specs: simplify bad argument checking with specl.badargs.

* specs/spec_helper.lua (badargs):  Expose specl.badargs to the
example execution environment.
(badarg, init): Remove.  Superseded by badargs.init.
* specs/debug_spec.yaml, specs/string_spec.yaml: Use badargs.init
instead of init.
* specs/functional_spec.yaml, specs/io_spec.yaml,
specs/list_spec.yaml, specs/math_spec.yaml,
specs/package_spec.yaml, specs/std_spec.yaml,
specs/table_spec.yaml: Use badargs.diagnose to write bad argument
diagnostics examples automatically.
* bootstrap.conf (buildreq): Use moonscript rocks server URLs.

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