Allow modern phd stop to stop old daemons cleanly

Summary:
Ref T7352. Make sure modern phd stop can still read the old PID file format and stop the daemons, at least for now.

Without this, stop still detects them and tells you to stop --force, which works, but this makes things a good deal cleaner.

Test Plan: Ran phd stop from master, then phd stop from this revision. Saw old daemons stop cleanly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7352

Differential Revision: https://secure.phabricator.com/D11873