ircuser: Fix setting of away message time by pre-0.13 cores

IrcUser::setLastAwayMessage() is called by pre-0.13 cores (newer
cores call setLastAwayMessageTime() instead). Fix wrong use of
QDateTime::from[M]SecsSinceEpoch(), which is a static method returning
a new QDateTime instance rather than a modifying setter.

Also fix signatures passing primitive types as const refs.