server: warn if user changes server port after init

So the user can view the current port number these cmds were changed to COMMAND_ANY.
However this means that the user can also attempt to change the port number after init,
even though this is not supported. Issue a warning that this is not supported.

Change-Id: I3d20dcd81277e7d994240a8e314f27672ff760c4
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/788


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>