mkfifoat: use new modules for Solaris and BSD bugs

mkfifoat: use new modules for Solaris and BSD bugs

Pick up Solaris 9 and BSD fixes to mkfifo and mknod.  No known
system has mknodat but broken mknod, so there is no need for
rpl_mkfifoat or rpl_mknodat.  Split mknodat into its own file.

* m4/mkfifoat.m4 (gl_FUNC_MKFIFOAT): Simplify.
* lib/mkfifoat.c (mknodat): Split...
* lib/mknodat.c (mknodat): ...into new file.
* modules/mkfifoat (Files): Ship new file.
(Depends-on): Add mkfifo, mknod.
* modules/mkfifoat-tests (Files): Reuse mkfifo tests.
(Depends-on): Add symlink.
* tests/test-mkfifoat.c (main): Enhance test.  Drop portions now
redundant with test_mkfifo.h.
(do_mkfifoat, do_mknodat): New helpers.

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