Update from Gnulib

Update from Gnulib

This incorporates:
2020-02-22 fchmodat, lchmod: simplify
2020-02-22 lchmod: fix link error on Solaris 10
2020-02-22 use 'restrict' in all POSIX function declarations
2020-02-22 chmodat, chownat: new modules
* lib/gnulib.mk.in: Regenerate.
* lib/inttypes.in.h, lib/openat.h, lib/signal.in.h:
* lib/stdio.in.h, lib/stdlib.in.h, lib/string.in.h:
* lib/sys_stat.in.h, lib/time.in.h, lib/unistd.in.h, m4/inttypes.m4:
* m4/signal_h.m4, m4/stdio_h.m4, m4/stdlib_h.m4, m4/string_h.m4:
* m4/sys_socket_h.m4, m4/sys_stat_h.m4, m4/time_h.m4:
* m4/unistd_h.m4: Copy from Gnulib.