zeroconf: notify the Zeroconf threads when hg exits
Zeroconf launches two threads in the background, and they wait on
Condition objects to exit. We need to call Zeroconf.close() to
release those conditions so that threads can gracefully exit.
This means that an interrupt on the hg process will now gracefully
propagate to the Zeroconf children, fixing that bug which did not
allow us to kill an `hg serve` process.
color: use ui.formatted() to test TTYness, not sys.stdout.isatty()
This fixes the color extension not working with pager (broken in
877390020477). The pager extension already sets ui.formatted=True to
allow this use case.