mkfifo-tests, etc.: allow HP-UX 11.11 bug

Problem reported by Daniel Richard G. in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-10/msg00068.html>.
* doc/posix-functions/mkfifo.texi (mkfifo):
* doc/posix-functions/mkfifoat.texi (mkfifoat):
* doc/posix-functions/mknod.texi (mknod):
* doc/posix-functions/mknodat.texi (mknodat):
Document the HP-UX 11.11 bug.
* tests/test-mkfifo.h (test_mkfifo):
Allow the HP-UX 11.11 bug.