this type is not really intended to be used; it's just there to allow
implementations to choose the type for the shm_nattch member of
struct shmid_sh, presumably since historical implementations disagreed
on the type. in any case, it needs to be there, so now it is.

this type was removed back in 5243e5f1 ,
because it was removed from the XSI specs.
however some apps use it.
since it's in the POSIX reserved namespace, we can expose it
unconditionally.

this fix is far from ideal and breaks the rule of not using
arch-specific #ifdefs, but for now we just need a solution to the
existing breakage.

the underlying problem is that the kernel folks made a very stupid
decision to make misalignment of this struct part of the kernel
API/ABI for x86_64, in order to avoid writing a few extra lines of
code to handle both 32- and 64-bit userspace on 64-bit kernels. I had
just added the packed attribute unconditionally thinking it was
harmless on 32-bit archs, but non-x86 32-bit archs have 8-byte
alignment on 64-bit types.

added various MS_*, MNT_*, UMOUNT_* flags following the linux
headers, with one exception: MS_NOUSER is defined as (1U<<31)
instead of (1<<31) which invokes undefined behaviour

the S_* flags were removed following glibc

using the glibc names for the magic constants of the linux reboot syscall

missing protocol families based on current linux headers:
PF_RDS, PF_LLC, PF_CAN, PF_TIPC, PF_NFC

these flags are needed in order to be able to handle lwp id's
which the kernel returns after clone() calls for new threads
via ptrace(PTRACE_GETEVENTMSG).

fortunately, they're the same for all archs and in the reserved
namespace.

the old definitions were wrong on some archs. actually, EPOLL_NONBLOCK
probably should not even be defined; it is not accepted by the kernel
and it's not clear to me whether it has any use at all, even if it did
work. this issue should be revisited at some point, but I'm leaving it
in place for now in case some applications reference it.

these are not specified in the standard, but in the reserved
namespace, so there is no problem with defining them unconditionally.

despite glibc using __key and __seq rather than key and seq, some
applications, notably busybox, assume the names are key and seq unless
glibc is being used. and the names key and seq are really the ones
that _should_ be exposed when not attempting to present a
standards-conforming namespace; apps should not be using names that
begin with double-underscore. thus, the optimal fix is to use key and
seq as the actual names of the members when in bsd/gnu source profile,
and define macros for __key and __seq that redirect to plain key and
seq.

traditionally, both BSD and GNU systems have it this way.
sys/syscall.h is purely syscall number macros. presently glibc exposes
the syscall declaration in unistd.h only with _GNU_SOURCE, but that
does not reflect historical practice.

previously the names were exposed as key/seq with _GNU_SOURCE and
__ipc_perm_key/__ipc_perm/seq otherwise, whereas glibc always uses
__key and __seq for the names. thus, the old behavior never matched
glibc, and the new behavior always does, regardless of feature test
macros.

for now, i'm leaving the renaming here in sys/ipc.h where it's easy to
change globally for all archs, in case something turns out to be
wrong, but eventually the names could just be incorporated directly
into the bits headers for each arch and the renaming removed.

the macro was the wrong way round, additionally GNU defines
__ prefixed versions, which are used by qemu.

this function is obsolete, however it's available as a syscall
and as such qemu userspace emulation tries to forward it to the
host kernel.

previously, a few BSD features were enabled only by _BSD_SOURCE, not
by _GNU_SOURCE. since _BSD_SOURCE is default in the absence of other
feature test macros, this made adding _GNU_SOURCE to a project not a
purely additive feature test macro; it actually caused some features
to be suppressed.

most of the changes made by this patch actually bring musl in closer
alignment with the glibc behavior for _GNU_SOURCE. the only exceptions
are the added visibility of functions like strlcpy which were BSD-only
due to being disliked/rejected by glibc maintainers. here, I feel the
consistency of having _GNU_SOURCE mean "everything", and especially
the property of it being purely additive, are more valuable than
hiding functions which glibc does not have.

these structures are purely for use by trace/debug tools and tools
working with core files. the definition of fpregset_t, which was
previously here, has been removed because it was wrong; fpregset_t
should be the type used in mcontext_t, not the type used in
ptrace/core stuff.

apparently some other archs have sys/io.h and should not break just
because they don't have the x86 port io functions. provide a blank
bits/io.h everywhere for now.

based on proposal by Isaac Dunham. nonexistance of bits/io.h will
cause inclusion of sys/io.h to produce an error on archs that are not
supposed to have it. this is probably the desired behavior, but the
error message may be a bit unusual.

this is mostly junk, but a few programs with tape-drive support
unconditionally include it, and it might be useful.

although a number is reserved for it, this option is not implemented
on Linux and does not work. defining it causes some applications to
use it, and subsequently break due to its failure.

the W* namespace is not reserved, so the nonstandard ones must be
moved under extension features. also WNOHANG and WUNTRACED were
missing.

it will be in the next version of POSIX