enh(io): use igtl::serverSocket to get real port number

Description

Allow the use of port 0, setting port = 0 will ask socket to find the first available port number. Thus we need to modify the way the port is stored in igtlNetwork::Server, and use igtl::ServerSocker::GetServerPort to have the real port number.

Closes #649 (closed)

How to test it?

Try your favorite App using IGTL, and try to set the server port to 0, it should check for a port, and then you can get it by using myserver->getPort();