Drive "phd stop" entirely from the process list, not PID files on disk

Summary:
Ref T13321. Depends on D20600. Make bin/phd stop mean:

• bin/phd stop: Stop all processes which have daemon process titles. If we're instanced, only stop daemons for the current instance.
• bin/phd stop --force: Stop all processes which have deamon process titles for any instance.

We no longer read or care about PID files on disk, and this moves us away from PID files.

This makes unusual flag --gently do nothing. A followup will update the documentation and flags to reflect actual usage/behavior.

This also removes the ability to stop specific PIDs. This was somewhat useful long, long ago when you might explicitly run different copies of the PullLocal daemon with flags to control which repositories they updated, but with the advent of clustering it's no longer valid to run custom daemon loadouts.

Test Plan: Ran bin/phd start, then bin/phd stop. Saw instance daemons stop. Ran bin/phd stop --force, saw all daemons stop.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13321

Differential Revision: https://secure.phabricator.com/D20601