llback argument).

Function will return as soon as all processes terminate or when
*timeout* occurs.
Differently from Process.wait() it will not raise TimeoutExpired if
*timeout* occurs.

Typical use case is:

 - send SIGTERM to a list of processes
 - give them some time to terminate
 - send SIGKILL to those ones which are still alive

Example:

>>> def on_terminate(proc):
...     print("process {} terminated".format(proc))
...
>>> for p in procs:
...    p.terminate()
...
>>> gone, alive = wait_procs(procs, timeout=3, callback=on_terminate)
>>> for p in alive:
...     p.kill()
c