Update the FSF's address in the licensing notices.

The FSF moved several years ago; this updates all the mentions of their
address in the GPL copy (for which I've just taken the latest plain-text
copy of the GPLv2 from the FSF web site) and the licensing headers in
our files.
Change pystatgrab's license to LGPLv2+.

We changed the license of libstatgrab to LGPLv2+ (rather than GPLv2+) in
April 2004, but we never updated pystatgrab to match. This fixes it.

The authors of pystatgrab are Tim Bishop and Adam Sampson; both are
happy with this change.