specl: diagnose unrecognised command-line options.

* specs/specl_spec.yaml: Specify behaviour for unrecognised
options.
* lib/specl/main.lua (process_args): Add parser option.  Use it
to diagnose unrecognised options.
(execute): Adjust accordingly.
* NEWS: Updated.

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