[mythtv] MythBackend and MythShutdown Poweroff, Wakeup commands.

[Matthew Wire]

Currently MythBackend and MythShutdown have separate settings for the
following:

Poweroff command
Reboot command
Set Wakeup Time command

I can't see any logical reason for having these separate since they are
per-host anyway.

Hence I propose merging them into one - speak up now if you have a valid
reason for keeping them separate.