[mythtv] Slave backend must start before master

[Brian Foddy]

A secondary problem I've found from the UNKNOWN_COMMAND problem,
I've also seen where the slave backend must be started before the master
for the system to work correctly.  Seems when the slave connects to a
already running master, the master does not immediately recognize the
new tuner cards in the slave.  Any programs depending on those
cards are not scheduled, and any attempt to watch TV on those
cards will result in a frontend error message about the backend not
being present.

Perhaps eventually they are recognized, but not right away.

My master has 2 air2pc cards, the slave has 3 pvr250s.  So
they don't have any source overlap between the two.

Brian