ffmpeg_opt: use codec private context in ffserver streams

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
ffserver_config: handle codec private options

This commit allows to set codec's private option.
As side effect, it also improves preset support.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavc/options_table: set min to -1 for timecode_frame_start

timecode_frame_start is set to -1 in avcodec_get_context_defaults3()
AVOptions API complains about it.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavc/options_table: add pixel_format and video_size options

Adding these options simplify ffm format implementation based on AVOption API.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/ffm: use AVOption API to store/restore stream properties

This is a generic solution that will not reqiore modifications when new options are added.
This also fixes problem with current implementation when qmin or qmax=-1.
Only 8 bits was sent and read back as 255.

Fixes #1275
Fixes #1461

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>