Update links in COPYING, COPYINGv3, and COPYING.EXCEPTION.

Update links in COPYING, COPYINGv3, and COPYING.EXCEPTION.

 - COPYINGv3 now exactly matches
   https://www.gnu.org/licenses/gpl-3.0.txt
   as of 2020-07-10.
   The only change was to turn a bunch of http:// URLs
   into https:// URLs.

 - COPYING now exactly matches
   https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt
   as of 2020-07-10
   except that the FSF’s old postal address has been deleted
   and <https://fsf.org/> inserted
   (consistent with COPYINGv3).
   Besides the above, the only change was to whitespace.

 - A typo in a URL in COPYING.EXCEPTION was corrected.