debug: support trailing "?" in place of separate "nil" in argcheck.

* specs/debug_spec.yaml (argcheck): Specify behaviour of trailing
"?" in type specifiers.
* lib/std/base.lua (argcheck): Strip trailing "?" from acceptable
argument types, appending a "nil" entry to the list if any "?"
is stripped.
* lib/std/base.lua, lib/std/debug.lua, lib/std/functional.lua,
lib/std/math.lua, lib/std/package.lua, lib/std/string.lua,
lib/std/table.lua: Adjust all callers to use trailing "?" instead
of separate explicit "nil" in type list.

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