lavc: Move b_frame_strategy and b_sensitivity to codec private options

The b_frame_strategy option is only used by mpegvideoenc, qsv, x264, and
xavs, while b_sensitivity is only used by mpegvideoenc.

These are very codec-specific options, so deprecate the global variants.
Set proper limits to the maximum allowed values.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
