Subtitles: fix missing CEA-708 closed captions.

This is a quick hack version of
7d9bf3193e0f3d1795c1ead6f16aa386ab7de587, which fixes a CEA-708
decoding bug exposed by recent changes to NBC's CEA-708 encoding.
(cherry picked from commit eca41c857ef9618a2e8335b90e97583e08858107)
Fix "mythcommflag --rebuild" for non-HDPVR recordings.

By default, only identify IDR keyframes for h.264 recordings.
However, since DVB-S recordings may lack IDR frames, we check after
1000 frames whether any keyframes have been found, and if not, back up
and restart using the more lenient setting.

Adapted from 802e32ba4dc8852da56db170893e95cbc28637e3 and
77bf3275ef3133d81fc979ae40d9728523ff9974.