fcntl: bind posix_fadvise minimally.

Close #169.
* specs/posix_spec.yaml (fadvise): Move from here...
* specs/posix_compat_spec.yaml (fadvise): ...to here.
* specs/posix_fcntl_spec.yaml (posix_fadvise): Specify argcheck
behaviours on supporting systems.
* ext/posix/fcntl.c (Pfadvise): Rename from this...
(Pposix_fadvise): ...to this.  Change first argument to a file
descriptor to match POSIX API.
* lib/posix/compat.lua (fadvise): Reimplement file handle
argument version in Lua.
* NEWS: Update.

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