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>