Calling nl_recvmsgs_default() leads dirctly to recvmsgs() from
"nl.c". This functions reads messages (recvmsg) in a loop and
invokes the callbacks.

Later we want to merge nlh and nlh_event, meaning that we must
anticipate parsing unrelated messages while waiting for an ACK.
While that would be possible by registering different callbacks
and letting them interact, it is actually more complicated. Just
assume full control over the message parsing.

Basically, copy recvmsgs() to event_handler_recvmsg(). For now just
copy the function and do little adjustment (to show the similarity
to the original). Cleanup follows.