posix: wrap stat and lstat correctly.

* specs/posix_spec.yaml (describe stat): Adjust specs for correct
behaviour of stat.
(describe lstat): New examples to specify behaviour of new lstat
wrapper.
* ext/posix/posix.c (Pstat): Call stat(2), not lstat(2)!
(Plstat): New wrapper for lstat(2).
(R): Add Plstat.
* NEWS: Update.

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