fchdir: improve use of replacement functions

fchdir: improve use of replacement functions

* m4/fchdir.m4 (gl_FUNC_FCHDIR): Set appropriate witnesses.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Add REPLACE_FSTAT.
* m4/dirent_h.m4 (gl_DIRENT_H_DEFAULTS): Add REPLACE_OPENDIR,
REPLACE_CLOSEDIR.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add REPLACE_DUP.
* modules/sys_stat (Makefile.am): Substitute correct witness.
* modules/dirent (Makefile.am): Likewise.
* modules/unistd (Makefile.am): Likewise.
* lib/dirent.in.h (opendir, closedir): Use better witnesses.
* lib/unistd.in.h (dup): Likewise.
* lib/sys_stat.in.h (fstat): Likewise.

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