ao_lavc: support non-interleaved audio
mf: silence compilation warning
ao_null: support pausing properly

ao_null should simulate a "perfect" AO, but framestepping behaved quite
badly with it. Framstepping usually exposes problems with AOs dropping
their buffers on pause, and that's what happened here.
ao_null: add untimed sub-option
m_option: handle audio/filter filters with old option parsing

These use the _oldargs_ hack, which failed in combination with playback
resume. Make it work.

It would be better to port all filters to new option parsing, but that's
obviously too much work, and most filters will probably be deleted and
replaced by libavfilter in the long run.