For --mua users writing to lock-free -o MFOLDER destinations;
we'll keep -WINCH and send an ASCII terminal bell when results
are complete. This is intended to let early MUA spawners know
when lei2mail is done writing results.
We'll also support running arbitrary commands. It may be used
to run play(1) (from SoX), handle pipelines+redirects
(e.g. "/bin/sh -c 'echo search done | wall'") or other commands.