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

src/docs/managing_daemons.diviner

@@ -34,6 +34,7 @@ a list of commands, run ##phd help##:
 Generally, you will use:
 
   - **phd launch** to launch daemons;
+  - **phd debug** to debug problems with daemons;
   - **phd status** to get a list of running daemons; and
   - **phd stop** to stop all daemons.
 
@@ -54,6 +55,10 @@ allows you to view log information for them. If you have issues with daemons,
 you may be able to find error information that will help you resolve the problem
 in the console.
 
+NOTE: The easiest way to figure out what's wrong with a daemon is usually to use
+**phd debug** to launch it instead of **phd launch**. This will run it without
+daemonizing it, so you can see output in your console.
+
 = Available Daemons =
 
 You can get a list of launchable daemons with **phd list**: