val add : Unix.file_descr -> ( (int * int) -> unit ) -> [ `Revert of unit -> unit ]
add fd f registers a
SIGWINCH handler. Every time the signal is delivered,
f is called with the current size of the tty backing
fd is not a tty,
f is never called.
Return value is a function that removes the handler
Handlers are called in an unspecified order.