audio/filters: add af_force

Its main purpose is for testing in case channel layout stuff breaks, in
particular in connection with old audio filters.
ao_openal: use channel map instead of ALSA fixed layout
manpage: update --channels
af_lavrresample: avoid channel reordering with unknown layouts

If one of the input or output is an unknown layout, but the other is
known, it can still happen that channels are remixed randomly. Avoid
this by forcing default layouts in this case. (Doesn't work if the
channel counts are different.)
audio: add channel map selection function
ao_pulse: move format setup code
audio/out: channel map selection

For now, alsa/pulse only. WIP.