badargs: helpers for argchecking specs.

* lib/specl/badargs.lua: New file.
(init):  Lua core style bad argument error message generator
factory.
(diagnosebadargs): Use new programmatic examples to inject specs
for expected bad argument diagnostics according to `specifier`
argument.
* local.mk (dist_specl_DATA): Add lib/specl/badargs.lua.
* NEWS: Update.

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