fdopendir: split into its own module

fdopendir: split into its own module

* lib/openat.c (fdopendir): Move...
* lib/fdopendir.c: ...into new file.
* modules/fdopendir: New module.
* m4/fdopendir.m4 (gl_FUNC_FDOPENDIR): New file.
* modules/openat (Depends-on): Add fdopendir.
* m4/openat.m4 (gl_FUNC_OPENAT): No longer need to check for
fdopendir here.
* modules/savedir (Depends-on): Only need fdopendir, not full
openat.
* lib/savedir.c (include): Use <dirent.h>, not "openat.h".
* lib/openat.h (fdopendir): Drop prototype.
* lib/dirent.in.h (fdopendir): Provide prototype.
* m4/dirent_h.m4 (gl_DIRENT_H_DEFAULTS): Add replacements.
* modules/dirent (Makefile.am): Substitute them.
* MODULES.html.sh (File system functions): Mention it.
* doc/posix-functions/fdopendir.texi (fdopendir): Likewise.
* modules/fdopendir-tests: New file.
* tests/test-fdopendir.c: Likewise.

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