align with commit c9c2cd3e.

PAGESIZE is actually the version defined in POSIX base, with PAGE_SIZE
being in the XSI option. use PAGESIZE as the underlying definition to
facilitate making exposure of PAGE_SIZE conditional.

use of MB_CUR_MAX encoded a hidden dependency on the currently active
locale for the calling thread, whereas nl_langinfo_l is supposed to
report for the locale passed as an argument.

general policy is that all source files defining a public API or an
ABI mechanism referenced by a public header should include the public
header that declares the interface, so that the compiler or analysis
tools can check the consistency of the declarations. Alexander Monakov
pointed out a number of violations of this principle a few years back.
fix them now.

add a member of appropriate type to the fpos_t union so that accesses
are well-defined. use long long instead of off_t since off_t is not
always exposed in stdio.h and there's no namespace-clean alias for it.

access is still performed using pointer casts rather than by naming
the union member as a matter of style; to the extent possible, the
naming of fields in opaque types defined in the public headers is not
treated as an API contract with the implementation. access via the
pointer cast is valid as long as the union has a member of matching
type.

previously this macro used an odd if/else form instead of the more
idiomatic do/while(0), making it unsafe against omission of trailing
semicolon. the omission would make the following statement conditional
instead of producing an error.

this is the idiom that's used elsewhere and should be more efficient
or at least no worse.

they seem to be relics of e3cd6c5c
where this code was refactored from a check that previously masked
against (F_ERR|F_NOWR) instead of just F_NOWR.

formally, calling readv with a zero-length first iov component should
behave identically to calling read on just the second component, but
presence of a zero-length iov component has triggered bugs in some
kernels and performs significantly worse than a simple read on some
file types.

the stdio FILE read backend's return type is size_t, not ssize_t, and
all of the special (non-fd-backed) FILE types already return the
number of bytes read (zero) on error or eof. only __stdio_read leaked
a syscall error return into its return value.

fread had a workaround for this behavior going all the way back to the
original check-in. remove the workaround since it's no longer needed.

replace with simple conditional that doesn't rely on assumption that
cnt is either 0 or -1.

the ':' in optstring has special meaning as a flag applying to the
previous option character, or to getopt's error handling behavior when
it appears at the beginning. don't also accept a "-:" option based on
its presence.

based loosely on patch by Hauke Mehrtens; converted to wrap the public
API of the underlying getrandom function rather than direct syscalls,
so that if/when a fallback implementation of getrandom is added it
will automatically get picked up by getentropy too.

This syscall is available since Linux 3.17 and was also implemented in
glibc in version 2.25 using the same interfaces.

signal context definitions for scalable vector extension new in commit
d0b8cd3187889476144bd9b13bf36a932c3e7952

it's a recent addition to elf gabi:
http://sco.com/developers/gabi/latest/revision.html
based on discussions at
https://sourceware.org/bugzilla/show_bug.cgi?id=15835

DF_1_STUB and DF_1_PIE were added in binutils-gdb commit
5c383f026242d25a3c21fdfda42e5ca218b346c8

NT_ARM_SVE and NT_S390_RI_CB are new in linux commits
43d4da2c45b2f5d62f8a79ff7c6f95089bb24656 and
262832bc5acda76fd8f901d39f4da1121d951222
the rest are older.
musl missed NT_PRFPREG because it followed the glibc api:
https://sourceware.org/bugzilla/show_bug.cgi?id=14890

allows calling extern functions without saving r2, for details see
glibc commit 0572433b5beb636de1a49ec6b4fdab830c38cdc5

AT_L1I_*, AT_L1D_*, AT_L2_* and AT_L3_* were added in linux v4.11 for
powerpc in commit 98a5f361b8625c6f4841d6ba013bbf0e80d08147.

PR_SVE_SET_VL and PR_SVE_GET_VL controls are new in linux commit
2d2123bc7c7f843aa9db87720de159a049839862
related PR_SVE_* macros were added in
7582e22038a266444eb87bc07c372592ad647439

HWCAP_SVE is new in linux commit 43994d824e8443263dc98b151e6326bf677be52e
HWCAP_SHA3, HWCAP_SM3, HWCAP_SM4, HWCAP_ASIMDDP and HWCAP_SHA512 are new in
f5e035f8694c3bdddc66ea46ecda965ee6853718

for systems without tp register or kuser helper, new in linux commit
8fcd6c45f5a65621ec809b7866a3623e9a01d4ed

PPC_FEATURE2_HTM_NO_SUSPEND is new in linux commit
cba6ac4869e45cc93ac5497024d1d49576e82666
PPC_FEATURE2_DARN and PPC_FEATURE2_SCV were new in v4.12 in commit
a4700a26107241cc7b9ac8528b2c6714ff99983d

to store hypervisor information, added in linux commit
3d8757b87d7fc15a87928bc970f060bc9c6dc618

new socekt option for AF_INET6 SOL_RAW sockets, added in linux commit
84e14fe353de7624872e582887712079ba0b2d56

TCP_FASTOPEN_KEY is new in 1fba70e5b6bed53496ba1f1f16127f5be01b5fb6
TCP_FASTOPEN_NO_COOKIE is new in 71c02379c762cb616c00fd5c4ed253fbf6bbe11b

for synchronous page faults, new in linux commit
1c9725974074a047f6080eecc62c50a8e840d050 and
b6fb293f2497a9841d94f6b57bd2bb2cd222da43
note that only targets that use asm-generic/mman.h have this new
flag defined, so undef it on other targets (mips*, powerpc*).

use the same token to define TIOCSER_TEMT as is used in ioctl.h
so when both headers are included there are no redefinition warnings
during musl build.

for querying tcp md5 signing keys.
new in linux commit c03fa9bcacd9ac04595cc13f34f3445f0a5ecf13

*_HUGE_SHIFT, *_HUGE_2MB, *_HUGE_1GB are documented in the man page,
so add all of the *_HUGE_* macros from linux uapi.

if MAP_HUGETLB is set, top bits of the mmap flags encode the page size.
see the linux commit aafd4562dfee81a40ba21b5ea3cf5e06664bc7f6

if SHM_HUGETLB is set, top bits of the shmget flags encode the page size.
see the linux commit 4da243ac1cf6aeb30b7c555d56208982d66d6d33

*_HUGE_16GB is defined unsigned to avoid signed left shift ub.

new ethertypes in linux v4.14:
ETH_P_ERSPAN new in 84e54fe0a5eaed696dee4019c396f8396f5a908b
ETH_P_IFE new in 2804fd3af6ba5ae5737705b27146455eabe2e2f8
ETH_P_NSH new in 155e6f649757c902901e599c268f8b575ddac1f8
ETH_P_MAP new in 7373ae7e8f0bf2c0718422481da986db5058b005

new in linux commmit cdf4969c42a6c1a376dd03a9e846cf638d3cd4b1

TRAP_BRANCH and TRAP_HWBKPT new in linux commit
da654b74bda14c45a7d98c731bf3c1a43b6b74e2

indicates ARMv8.2-DCPoP persistent memory support extension.
new in linux commit 7aac405ebb3224037efd56b73d82d181111cdac3

allows zeroing anonymous private pages inherited by a child process.
new in linux commit d2cd9ede6e193dd7d88b6d27399e96229a551b19

MSG_ZEROCOPY socket send flag avoids copy in the kernel
new in linux commit 52267790ef52d7513879238ca9fac22c1733e0e3
SO_ZEROCOPY socket option enables MSG_ZEROCOPY if availale
new in linux commit 76851d1212c11365362525e1e2c0a18c97478e6b

socket option for kernel TLS support
new in linux commit 3c4d7559159bfe1e3b94df3a657b2cda3a34e218