This adds a service to run a tcpdump during the run.  This can be
useful to capture various network traffic for post analysis.

There didn't seem to quite be an appropriate place to document it, so
a new debugging file is started, with some terse explaination of our
various system-wide debugging services.

Change-Id: I09aaa57611c5047d09a9bce7932d34e9d50b30e6