This reverts commit 28411652.

The problem is that (1) we don't really support multi-homing, but (2) on
some system, if the other IP addresses that the operating system
considers part of the multi-homed connection are HEARTBEAT'ed (e.g.,
docker), they answer with SCTP ABORT, which breaks the entire
connection.

In other words, force single-homing by binding. For F1-U uses, another
option will be introduced that allows to bind to a separate interface
for F1-U (separate from F1-C), if necessary.

Closes #727