debug: add file object matching to argcheck.

* specs/debug_spec.yaml (argcheck): Specify behaviour when
matching against a file type argument.
* lib/std/debug.lua (argcheck): Add LDocs for file type.
* lib/std/base.lua (argcheck): When the required type is "file"
use io.type () to ensure that an open file object argument was
passed.

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