fnmatch: wrap fnmatch(3) properly.

Close #168.
* specs/posix_spec.yaml (fnmatch): Move from here...
* specs/posix_compat.yaml (fnmatch): ...to here.
* specs/posix_fnmatch_spec.yaml (fnmatch): Specify correct thin
binding for fnmatch(3).
* specs/specs.mk (specl_SPECS): Add posix_fnmatch_spec.yaml.
* ext/posix/fnmatch.c (fnmatch): Return the actual result of
the call.
(luaopen_posix_fnmatch): Export FNM_NOMATCH constant.
* lib/posix/compat.lua (fnmatch): Rewrite the weird true/false
error raising API here for backwards compatibility.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>