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>

src/server/server.c

@@ -635,6 +635,10 @@ SERVER_PIPE_COMMAND()
 			break;
 		case 1:
 		{
+			if (CMD_CTX->mode == COMMAND_EXEC) {
+				LOG_WARNING("unable to change server port after init");
+				return ERROR_COMMAND_ARGUMENT_INVALID;
+			}
 			const char *t = strdup(CMD_ARGV[0]);
 			free((void *)*out);
 			*out = t;