badargs: specify format behaviours.

* specs/badargs_spec.yaml: New file.  Specify format behaviours.
* specs/specs.mk (specl_SPECS): Add badargs_spec.yaml.
* lib/specl/badargs.lua (format): Only return "argument" for
exactly one argument, otherwise "arguments".
* NEWS: Update.

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