Improve handling of local ports

[RussellJoyce]

Currently if two sessions are started from the same machine with the same port (even with two different users) then the second session will fail to create the SSH tunnel but will appear to have connected nonetheless. This can lead to multiple users hijacking one session due to everyone using the default port (12345) and then connecting to this in Vivado/SDK.

While users can specify a custom port using the -l option to the client script, this needs to be clearer as an option, and if the port cannot be used then the script needs to fail rather than carrying on regardless.

Using a random default port would help with this, but then the VLAB connection in the Xilinx tools will have to be modified frequently, which is likely to cause problems.

An alternative could be to assign (or calculate) a different default port for each user, or to just encourage (or force) people to specify a custom port when connecting.

[iangray001]

I improved the detection and error reporting in the client script, but this is still all a bit of a hack.

Unfortunately, because Vivado needs to be told the port to connect on, entirely random ports does not help here.