Added Affero-like provision, with definition of 'Network Service'.

This provision incorporates the AGPLv3 interpretation of RMS and
bkuhn, according to which a mere distributor of a modified version
must design the software in such a way that, when run to provide a
network service, it reasonably can be expected to inform users how to
get the source code (in addition to the requirement applying to the
mere network-service-deployer of a version modified by the deployer).