[mythtv] FIX "do not autoexpire" Flag made persistent

[J. Donavan Stanley]

Chris Pinkham wrote:

>>similar is in Myth. And if someone who know what he is doing also have the
>>possibility to ad this feature to his CVS. I did not want to step on anyones
>>toe or offend someone, I was only learning to understand Myth and add a feature
>>I already missed once ore twice.
>>    
>>
> My sole intention was to add this feature for me as long as not anything
>
>
>Sorry if I came across that way.  I'd like to see the undeletable feature
>added, but not by overloading the autoexpire field.
>  
>

What if we just inverted the flag so that it indicates "Keep this until 
I delete it".  Then it's just a matter of a dbCheck update and some UI 
text for the existing installations.