refactor: use toomanyargmsg function instead of toomanyarg_fmt string.

* lib/std/debug.lua (toomanyarg_fmt): Remove.
(toomanyargmsg): A replacement function that returns the formatted
string.  Adjust all callers.
* specs/debug_spec.yaml (extend_base): Adjust accordingly.

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