R1:d98bf8ef8ee6

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.