Consider the --interface argument when broadcasting for scheduler discovery

[torokati44]

This PR aims to (at least partially) solve #538.
It does not add any filtering for incoming packets, but at least the daemon won't look for schedulers on any interface other than the one given as command line argument (if one is given...).
It might not be perfect yet, because the naming of the new variables might be confusing here and there, and the value of the --interface option has to be passed around a lot in all sorts of places, but I'm open to suggestions for improvement.
Maybe adding some comments would be enough to explain what the new variables/parameters are there for?
Also, this might break icemon.

[llunak]

The patch in principle looks good, but can you please make sure it's backwards source compatible (i.e. that icemon compiles without changes)? In some cases it's not necessary at all to pass the extra argument (e.g. DiscoverSched::sendSchedulerDiscovery() can use the member variable), in other cases it can be the last argument with a default value (which mostly will be also better API, with the argument not being very important in the argument list).