ffserver_config: add ffserver_free_config

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

Conflicts:
	ffserver_config.c
lavu/opt: introduce av_opt_is_set_to_default()

TODO: bump minor version, update doc/APIchanges

New function allows to check if option is set to its default value

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

Conflicts:
	libavutil/opt.c
	tests/ref/fate/opt
lavu/opt: introduce av_opt_serialize()

TODO: bump minor version, update doc/APIchanges

Function allows to create string containing object's serialized options.
Such string may be passed back to av_set_options_string() in order to restore options.

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

Conflicts:
	libavutil/opt.c
ffserver: allow skip setting defaults

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

Conflicts:
	ffserver_config.c
	ffserver_config.h
test

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

Conflicts:
	libavcodec/options_table.h
compilation fix
lavc/options: initialize pkt_timebase

It's default in option_table.h is 0, but without this fix it is represented as 0/0.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavc/options: set timecode_frame_start to -1 as option default

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>

Conflicts:
	libavcodec/options_table.h