posix: move mode munching from C to Lua.

Close #147.
* specs/posix_spec.yaml (chmod, mkdir, mkfifo, msgget, open):
Move from here...
* specs/posix_compat_spec.yaml (chmod, mkdir, mkfifo, msgget)
(open): ...to here, and add functionality examples.
* specs/posix_sys_msg_spec.yaml (msgget),
specs/posix_sys_stat_spec.yaml (chmod, mkdir, mkfifo): Updated
versions of original examples, tailored to work with integer mode
arguments.
* local.mk (specl_SPECS): Add specs/posix_fcntl_spec.yaml and
specs/posix_sys_msg_spec.yaml.
* ext/posix/fcntl.c (Popen): Drastically simplified, to a
minimal POSIX binding.
* ext/posix/sys/msg.c (Pmsgget): Likewise.
* ext/posix/sys/stat.c (Pchomd, Pmkdir, Pmkfifo): Likewise.
* ext/include/_helpers.c (M, rwxrwxrwx, octal_mode, mode_munch):
Remove.  No longer in use.
* lib/posix/compat.lua (checkint): New support function.
(chmod, mkdir, mkfifo, msgget, open): Reimplement legacy mode
munching APIs over minimal C bindings.
(create): Adjust to use new minimal open binding.

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