ccanlint: add simple check for comment referring to LICENSE file.

ccanlint: add simple check for comment referring to LICENSE file.

After discussion with various developers (particularly the Samba
team), there's a consensus that a reference to the license in each
source file is useful.  Since CCAN modules are designed to be cut and
paste, this helps avoid any confusion should the LICENSE file go
missing.

We also detect standard boilerplates, in which case a one-line summary
isn't necessary.