fchmodat, fchownat, fstatat: use extern-inline

fchmodat, fchownat, fstatat: use extern-inline

* lib/fchmodat.c, lib/openat.h (FCHMODAT_INLINE):
* lib/fchownat.c, lib/openat.h (FCHOWNAT_INLINE):
* lib/fstatat.c, lib/openat.h (FSTATAT_INLINE):
New macros.
* lib/openat.h:
Replace all uses of 'static inline' with them.
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* modules/fchmodat, modules/fchownat, modules/fstatat:
* modules/openat-h:
(Depends-on):
Add extern-inline.
(configure.ac): Remove AC_C_INLINE.