Allow "phage" to print execution status on SIGINT

Summary: Ref T13490. The new Arcanist runtime supports workflow signal handling, but Phage isn't quite able to make use of it. Clean up the last few pieces so it can work.

Test Plan: Ran "phage", hit ^C, got status information.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21060