helpers: separately determine if stdout and stderr are ttys.

helpers: separately determine if stdout and stderr are ttys.

Previously we only cared if stderr was a tty (since we use that to determine
if we should print progress() or not).  But we might want to check stdout as
well, for the same reason that gzip does: we should be refusing to write
binary data to a terminal.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>