Initial implementation of MP4 DTCP support

Also found/fixed a bug in the TimeSeekRange response when protection is not
 enabled. It was passing end_point.byte_offset-1 instead of
 end_point.byte_offset. The byte offsets from the MP4 AccessPoints are
 non-inclusive.

Tested with LPTT. No unexpected failures.
Merge branch 'cablelabs/master' into cbl_cpratt/mp4-time-seek.seg