Use server.port=0 for scanning

This leverages existing capabilities of teh JDK and the OS
to grab a port at random and not have it stolen by another
process. It's very hard to avoid that race condition in
pure Java code, so why bother?

User can set port<0 to disable autoStart of connectors (e.g.
to start a web application context but not have it listen on
any port). In that case the actual socket port will be set to
0 (and therefore if it ever starts up the local port will
be random).