Fixes #746 - Quassel unsets default modes set by server upon first connect

Fixes #746 - Quassel unsets default modes set by server upon first connect
Quassel now uses a saner approach and only keeps track of the mode changes you
really issued and also were confirmed by the server.
You can reset the auto restore modes via "/mode -reset". This command gives
currently no userfeedback as it would introduce new strings and this patch
is intended to be backported.