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*).

*_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.

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

allows the os to free the marked pages lazily on memory pressure.
expected to increase malloc performance.
new in linux commit 854e9ed09dedf0c19ac8640e91bcc74bc3f9e5c9

currently five targets use the same mman.h constants and the rest
share most constants too, so move them to sys/mman.h before the
bits/mman.h include where the differences can be corrected by
redefinition of the macros.

this fixes two minor bugs: POSIX_MADV_DONTNEED was wrong on most
targets (it should be the same as MADV_DONTNEED), and sh defined
the x86-only MAP_32BIT mmap flag.

they lock faulted pages into memory (useful when a small part of a
large mapped file needs efficient access), new in linux v4.4, commit
b0f205c2a3082dd9081f9a94e50658c5fa906ff1

MLOCK_* is not in the POSIX reserved namespace for sys/mman.h

the definitions are generic for all kernel archs. exposure of these
macros now only occurs on the same feature test as for the function
accepting them, which is believed to be more correct.

both the kernel and glibc agree that this argument is unsigned; the
incorrect type ssize_t came from erroneous man pages.

patch by Justin Cormack, with slight modification

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

this is ugly and stupid, but now that the *64 symbol names exist, a
lot of broken GNU software detects them in configure, then either
breaks during build due to missing off64_t definition, or attempts to
compile without function declarations/prototypes. "fixing" it here is
easier than telling everyone to add yet another feature test macro to
their builds.

patch by Isaac Dunham. matched closely (maybe not exact) to glibc's
idea of what _BSD_SOURCE should make visible.