[mythtv] Add IsMasterBackend and BackendIsRunning functions to mythcontext

[Matthew Wire]

See http://svn.mythtv.org/trac/ticket/3280


Moves BackendIsRunning? and adds new IsMasterBackend? function to
mythcontext.

These both return boolean values and are used in mythbackend and
mythtv-setup. They are also useful elsewhere and this makes them
available to every app utilising mythcontext.