Merge from Gnulib; use ‘open’ for O_CLOEXEC

Merge from Gnulib; use ‘open’ for O_CLOEXEC

This incorporates:
2017-08-15 renameat: ensure declaration in <stdio.h> on NetBSD
2017-08-15 extensions: enable NetBSD specific extensions
2017-08-14 open: support O_CLOEXEC
2017-08-13 reallocarray: new module
* admin/merge-gnulib (AVOIDED_MODULES): Remove ‘open’, since
it now supports O_CLOEXEC and this simplifies Emacs.
* build-aux/config.guess, lib/fcntl.in.h, lib/stdio.in.h:
* lib/stdlib.in.h, m4/extensions.m4, m4/stdlib_h.m4:
Copy from Gnulib.
* lib/cloexec.c, lib/cloexec.h, lib/open.c:
* m4/mode_t.m4, m4/open-cloexec.m4, m4/open.m4:
New files, copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib-src/etags.c (O_CLOEXEC) [WINDOWSNT]:
Remove, as Gnulib does this for us.
* src/filelock.c (create_lock_file):
* src/sysdep.c (emacs_open, emacs_pipe):
Don’t worry about O_CLOEXEC == 0, as Gnulib no longer sets it to 0.