Add multi socket support

This commit adds support for multiple listening sockets per serivce.

This allows listening on muliple sockets/ports for each given
service. It is especially usefull in transitional IPv4/IPv6 setups
w/ specific needs for socket bindings.

This commit changes the config handling slightly by differntiating
between set_config_key and add_config_key. The keywords which define
a port binding are feed to the new add_config_key methods
while the old set_config_key methods behave exactly as before.

It will throw out a wildcard c2s bind on port 5222 if there
are any subsequent c2s ports defined. This necessary since
we can not bind to one port twice.

It also introduces some new testcases for the new feature.