Send extra modes via non-breaking protocol change

Send extra user mode updates in backwards-compatible manner.

When multiple modes are received in IrcChannel::joinIrcUsers, call
addUserMode one time for each, then call ircuser->addUserModes() with
the new modes.  Perhaps less efficient, but avoids breaking protocol.

Reduce excess network traffic by only syncing IrcUser::setUserModes
and IrcUser::addUserModes when changes are made.

Preliminary test results on Ubuntu (more testing always welcome!)
* New core, new client - works fine (as one might hope)
* New core, old client - client prints warning to console but works
(IrcChannel modes might internally get out of sync, but visible
 channel list updates with mode changes, multiple modes, etc)
* New core, Quasseldroid alpha/NG - works fine
* Old core, new client - works fine

If desired, the warning and other potential risks could be avoided
entirely by implementing a new Feature flag and only sending the
additional mode information to clients that request it.  However,
this seemed excessive to avoid what in limited testing was only a
warning in the client console log, normally hidden away from sight.