lavc: Move frame_skip_* to codec private options

lavc: Move frame_skip_* to codec private options

These options are only used by mpegvideoenc and vpx.
They are very codec-specific options, so deprecate the global variants.

Add an allowed value to the private options for frame_skip_cmp which
seems to have been forgotten, but perfectly working.

The libvpx frame dropping feature uses one of such option
(frame_skip_threshold) without the other three. For this reason rename
the option to something more consistent with the other libvpx variables.

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