Make parsing of daemon PIDs more robust

Summary:
In production on the cluster, we get output like this:

  1 process1
 22 process2
333 process3

That is, the PIDs are right-aligned. In this case, the explode(...) splits the line " 1 process1" on the first space, and we end up with an empty PID.

We later try to kill PID 0, which kills us.

Instead, trim.

Also, be slightly more liberal in splitting.

Test Plan: Ran phd stop --force in production, saw daemons get killed instead of the process itself.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11786