Support listening on port 0.

Support listening on port 0.

When listening on port 0, the system will assign a random open port. We
use this to run multiple OpenOCD instances against multiple simulators
as part of regression testing. This mechanism means the various test
instances don't have to coordinate to ensure they don't reuse any ports.

The required changes are minimal:
1. Don't increment the port number when it's 0.
2. Print out which port was assigned by the system.

Change-Id: I404c801fc405e9d8eb8420562c02e78d4db6242f
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4316
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>