Fix RPL_ISUPPORT handling when the server has no MOTD

This is a quick hotfix for the problem that no network information is
processed at all if/when the server happens to send ERR_NOMOTD instead
of RPL_MOTDSTART. A proper fix would be not to rely on the MOTD messages
at all...
Merge branch '3.0'
Cleanup tst_IrcBufferModel::testChanges()
Revise IrcChannel::active handling

Make sure the channel is not considered active after re-connecting, but
before re-joining the channel. In other words, keep it inactive until
the channel has been joined again.
Fix IrcBufferModel::restoreState() to use existing buffers if available
WIP: IrcBufferModel: support multiple connections