lavu/opt: add av_opt_set_dict2() function

Existing av_opt_set_dict doesn't accept flags.
It doesn't allow to pass options to nested structs.
New function alllows that.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/mux: pass options to nested structs of priv data

This commit allows to benefit from implementing child_next
callback for muxers' AVClasses.
Without that, options cannot be set in nested structs.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavd/xv: reident after previous commits

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