servers: multihoming: Adapt endpoint registering to the introduced config parameters.

With this commit there are different possibilities to determine the endpoints
that will be propagated at the DIR.

1. multihomed.enabled is set to true and hostname is empty:
   Every valid address will be used with the corresponding network.
   It will be attempted to find the local host automatically and then use it as
   the default.

2. multihomed.enabled is set to true and hostname is set:
   As in the first case, but the configured hostname will be used to determine
   the default.

3. multihomes.enabled is not set and neither hostname nor listen.address
   It will be attempted to find the first non-local address and use it as a
   single endpoint matching any network. If this isn't successful it will be
   attempted to use the first local address. If this fails, too, the localhost
   will be used.

4. multihome.enabled is not set but hostname or listen.address
   The configured hostname or if not set listen.address will be used to as a
   single endpoint matching any network.