io: use argcheck for api functions.

* specs/io_spec.yaml (catdir, catfile, die, monkey_patch)
(process_files, readlines, shell, slurp, splitdir, warn)
(writelines): Specify argument type mismatch messages.
* lib/std/io.lua (catdir, catfile, die, monkey_patch)
(process_files, readlines, shell, slurp, splitdir, warn)
(writelines): Call argcheck to validate arguments and meet
specifications.

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