Allow callback server to open on a random port and override from the system property

currently the client can only use the port defined for <SOCKETSERVER> in the btconfig.xml and multi clients can not run together.

we need to allow callback server to open the random port when <SOCKETSERVER PORT="0"> or a range <SOCKETSERVER "12340-12350"> and they could be overridden by socketserver_port and socketserver_host environment variables.