maint: revert a1f9049 'move creat from Lua to C'.

* ext/posix/fcntl.c (creat): Remove.
* lib/posix.lua (creat): Implement in Lua.
* specs/posix_spec.yaml (creat): Adjust.
* NEWS: Update.

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