Make phd more aware of multiple daemons under a single overseer

Summary: Ref T7352. This makes phd stop and phd status produce more reasonable output with the new PID file format.

Test Plan: Ran phd stop, phd status, etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7352

Differential Revision: https://secure.phabricator.com/D11856