Update FSF address in COPYING file

This updates the GPLv2 license text with a few small tweaks, most
notably the new postal address for the FSF, which stops some package
linting tools complaining about the old address.

Patched directly via: `curl http://www.gnu.org/licenses/gpl-2.0.txt > COPYING`

(the empty line changes in the diff are deleted formfeed characters)