moodns: no need to actually wait for goroutines termination
assorted clean-up
moodns*: use Fatalf() instead of Fatal()

This removes the additional white spaces printed after the ":".
moodns.service: start after network stack
moodns.service: also, actually install the service
mdns: join/leave the multicast group whenever a new interface is brought up/down

This makes it possible to start moodns before a network interface has been
configured without getting an error from JoinGroup().

Additionally, it's now possible to switch to a different network interface (e.g.
from wired to wireless) without the need to restart moodns.

However, this currently only works on Linux (not that moodns worked on other
platforms before....).