FSC server port selected by argument

Discussed for example in https://issues.scala-lang.org/browse/SI-4504

Logic is following: If the existing -server argument has form localhost:port and there is no server running, then a new server is started at the given port. Therefore fsc can be used as without the server argument but fixing the port where to connect and start. If server argument points to another host server is not started.
Could be a `val`.