[mythtv] Support for toggle power / PACE550 in 6200ch channel changer

[Bryan Mayland]

I've submitted a ticket #4924 to add support for my cablebox (a PACE 
550) and add the ability to toggle the power from the utility.
http://svn.mythtv.org/trac/ticket/4924

The box returns "not implemented" when queried for power state, so myth 
can't intelligently set the power state.  Best thing I can do is check 
and see if I'm getting anything from it, and if not just toggle the 
power and try again. 

I know this is multiple changes in one patch but adding the pace box is 
adding 2 constants so really there's nothing to it.  If it needs to be 
broken in two I can do that I suppose.  I'm posting to the list for 
discussion of any reasons someone would have to not include this.