faccessat: new module

faccessat: new module

* modules/faccessat: New file.
* lib/faccessat.m4: Likewise.
* m4/faccessat.m4 (gl_FUNC_FACCESSAT): Likewise.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witness.
* modules/unistd (Makefile.am): Use it.
* lib/unistd.in.h (faccessat): Declare it.
(F_OK, X_OK, W_OK, R_OK): Provide definitions.
* lib/fcntl.in.h (AT_SYMLINK_FOLLOW, AT_EACCESS): Likewise.
* MODULES.html.sh (File system functions): Mention it.
* doc/posix-functions/faccessat.texi (faccessat): Likewise.
* doc/posix-headers/fcntl.texi (fcntl.h): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>