Copyright updates - update to GPLv3, etc.

Update to GPLv3.  secnet as actually installed is GPLv3+ anyway
because it depends on python-ipaddr (Apache 2.0, which is
GPLv2-incompatible), adns (now GPLv3+), and libgmp (now LGPLv3+).

Also:
* Add missing copyright notices and credits.
* Get rid of old FSF street address; use URL instead.
* Remove obsolete LICENCE.txt (which was for snprintf reimplementation).
* Remove obsolete references to Cendio (for old ipaddr.py, now gone).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>