these were leftover from early beginnings when arpa/inet.h was not
including netinet/in.h.

this is added for POSIX-future as the outcome of Austin Group issue
599. since it's in the reserved namespace for pthread.h, there are no
namespace considerations for adding it early.

_POSIX_VDISABLE is only visible if unistd.h has already been included,
so conditional use of it here makes no sense. the value is always 0
anyway; it does not vary.

this is not necessary for linux but is a simple, inexpensive change to
make that facilitates ports to systems where NAME_MAX needs to be
longer.

This macro exists since Linux 2.6.25 and is defined in glibc since 2011.

This enables alternative compilers, which may not define __GNUC__,
to implement alloca, which is still fairly widely used.

This is similar to how stdarg.h already works in musl; compilers must
implement __builtin_va_arg, there is no fallback definition.

this change was discussed on the mailing list thread for the linux
uapi v5.3 patches, and submitted as a v2 patch, but overlooked when I
applied the patches much later.

revert commit f291c09ec90e2514c954020e9b9bdb30e2adfc7f and apply the
v2 as submitted; the net change is just padding.

notes by Szabolcs Nagy follow:

compared to the linux uapi (and glibc) a padding is used instead of
aligned attribute for keeping the layout the same across targets, this
means the alignment of the struct may be different on some targets
(e.g. m68k where uint64_t is 2 byte aligned) but that should not affect
syscalls and this way the abi does not depend on nonstandard extensions.

allows waiting on a pidfd, in the future it might allow retrieving the
exit status by a non-parent process, see

  linux commit 3695eae5fee0605f316fbaad0b9e3de791d7dfaf
  pidfd: add P_PIDFD to waitid()

tcpi_rcv_ooopack for tracking connection quality:

  linux commit f9af2dbbfe01def62765a58af7fbc488351893c3
  tcp: Add TCP_INFO counter for packets received out-of-order

tcpi_snd_wnd peer window size for diagnosing tcp performance problems:

  linux commit 8f7baad7f03543451af27f5380fc816b008aa1f2
  tcp: Add snd_wnd to TCP_INFO