Allow the server host to specify its public ip address

Change the behavior of the masterserver of retrieving the host public by its own to check beforehand if the host specified its public address

and add a command line parameter(-publicip) for the host to specify its public ip

BTW, These changes don't break compatibility with clients using srb2 versions inferior to connect with the master server.

Before the merge request, the masterserver would retrieve the public address of the host without consulting the host, and that is a issue because some hosts(like me) owns a modem with unusually complex NATs where conventional port forwarding methods don't work.

For example, my Internet has actually two ip public addresses, one is assigned by default to all the devices connected to my network, this ip is practically impossible to port forward, BUT, I have figured my Internet has another ip, defined internally in the modem, this is the ip which is visible to the outside Internet and the port forward method works.