gnulib-tool: Verify that the License field is present and non-empty.

gnulib-tool: Verify that the License field is present and non-empty.

* gnulib-tool (func_get_license_raw): New function, extracted from
func_get_license.
(func_get_license): Use it. Warn if the module is not a test module and
has no license.
Suggested by Jim Meyering.