Allow any pending output from subprocesses to be read by Emacs.
It is read into the process' buffers or given to their filter functions.
Non-nil arg PROCESS means do not return until some output has been received
Non-nil second arg SECONDS and third arg MILLISEC are number of seconds
and milliseconds to wait; return after that much time whether or not
there is any subprocess output. If SECONDS is a floating point number,
it specifies a fractional number of seconds to wait.
The MILLISEC argument is obsolete and should be avoided.
If optional fourth arg JUST-THIS-ONE is non-nil, only accept output
from PROCESS, suspending reading output from other processes.
If JUST-THIS-ONE is an integer, don't run any timers either.
Return non-nil if we received any output before the timeout expired. (fn &optional PROCESS SECONDS MILLISEC JUST-THIS-ONE)