New license-files field.

Based closely on the patches by Mathieu Boespflug <mboes@cs.mcgill.ca>

This field is intended to be used instead of (or in addition to) the
normal 'license-file' field by packages that have multiple files for
their license material. This is useful when eg the license is
supplemented by additional permissions and/or conditions. Notably,
the LGPL is structured in this way: it amends the GPL with additional
permissions, therefore one should distribute both the GPL in COPYING
and the LGPL in COPYING.LESSER.

So we keep both the license-file and license-files fields (rather than
deprecating one) and packages can use either or a mixture.
Merge pull request #1642 from dcoutts/mboes-master

New license-files field.