don't allow "control mode" sessions to force resizing of other attached
client windows.
The default size is 80x24 which presumably overridden when
starting/attaching a client. If you want to attach via control mode to a
control, then 80x24 will be smaller and the resizing will take effect
limiting the size of the client window. It's important (to me) that this
not happen so sessions can be controlled externally without any visible
effects.