No STDIN for workers when not running interactively

When creating a pool with more than one worker, it doesn't make sense to
dup() STDIN to all these workers. Suppose there's four workers and all
of them want to read from STDIN -- who gets the input?

As a side effect, without this commit, terminals on Linux are "broken"
after a parallel "bw apply": echoing is turned off after bw has exited.
(Needs a "stty echo" or "reset" to get the terminal back.)