fchdir: use more consistent macro convention

fchdir: use more consistent macro convention

* lib/fcntl.in.h (_gl_register_fd): Move declaration to unistd.
* lib/sys_stat.in.h (rpl_fstat): Declare via make-time
REPLACE_FCHDIR, rather than relying on config.h macros.
* lib/unistd.in.h (fchdir): Move all fchdir internal declarations
inside a single make-time REPLACE_FCHDIR block, rather than using
the config.h FCHDIR_REPLACEMENT.
* m4/fchdir.m4 (gl_FUNC_FCHDIR): REPLACE_FCHDIR was already
AC_SUBST'd, also AC_DEFINE it.  Don't define FCHDIR_REPLACEMENT.
Manage fstat replacement.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Pick up
REPLACE_FCHDIR.
* modules/sys_stat (Files): Add m4/unistd_h.m4.
(Makefile.am): Substitute REPLACE_FCHDIR.
* lib/close.c (rpl_close): Use REPLACE_FCHDIR, not
FCHDIR_REPLACEMENT.
* lib/dup-safer.c (dup_safer): Likewise.
* lib/dup2.c (rpl_dup2): Likewise.
* lib/dup3.c (rpl_dup3): Likewise.
* lib/open.c (rpl_open): Likewise.

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