Re: File descriptors in exec'd subprocesses

I pushed the libpq changes.  I'll leave the pipe2 and accept4 changes
on ice for now, maybe for a later cycle (unlike the committed patches,
they don't currently fix a known problem, they just avoid some
syscalls that are already fairly rare).  For the libpq change, the
build farm seems happy so far.  I was a little worried that there
could be ways that #ifdef SOCK_CLOEXEC could be true for a build that
might encounter a too-old kernel and break, but it looks like you'd
have to go so far back into EOL'd releases that even our zombie build
farm animals have it.  Only macOS and AIX don't have it yet, and this
should be fine with Apple's availability guards, which leaves just
AIX.  (AFAIK headers and kernels are always in sync at the major
version level on AIX, but if not presumably there'd have to be some
similar guard system?)