debug: add argcheck APIs.

* specs/debug_spec.yaml (argcheck, argerror, argscheck): Specify
behaviour of argument checking functions based on luaL_argerror
and luaL_argcheck, for standardised argument error diagnostics.
* lib/std/debug.lua: Tidy up LDocs.
(argcheck, argerror, argscheck): New functions to satisfy the
specifications.
* NEWS: Update.

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