Add a "phd debug" command

Summary:
Make it easier to find obvious problems in daemons by letting them run
undaemonized in the console without requiring the user to know the magical
incantations for loading libraries, etc.

Test Plan:
Ran "phd debug nice", simulated some failures (e.g., bringing down Phabricator,
daemon fatal) and got useful error messages.

Reviewed By: jungejason
Reviewers: toulouse, jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 448