mkfifo: new module

mkfifo: new module

Solaris 9 mkfifo("name/",mode) mistakenly creates "name".
FreeBSD 7.2 mkfifo("dangling/",mode) mistakenly creates a fifo
at the target of "dangling".  Mingw lacks named pipes altogether,
but this at least avoids link failures.

* modules/mkfifo: New file.
* m4/mkfifo.m4 (gl_FUNC_MKFIFO): Likewise.
* lib/mkfifo.c (mkfifo): Likewise.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Set witness
defaults.
* modules/sys_stat (Makefile.am): Substitute them.
* lib/sys_stat.in.h (mkfifo): Declare replacement.
* MODULES.html.sh (Support for systems lacking POSIX:2008):
Document it.
* doc/posix-functions/mkfifo.texi (mkfifo): Likewise.
* modules/mkfifo-tests: New test.
* tests/test-mkfifo.h (test_mkfifo): New file, borrowed in part
from test-mkfifoat.c.
* tests/test-mkfifo.c: New file.

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