Copyright updates:
* Update to GPLv3. 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).
Copyright updates:
* Update to GPLv3. 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).