audio: improve mp_chmap_to_lavc_unchecked() unknown chmap behavior

Change it so that it will always return a bitmask with the correct
number of channels set if an unknown channel map is passed. This didn't
work for channel counts larger than 8, as there are not any standard
channel layouts defined with more than 8 channels (both in mpv and
FFmpeg). Instead, it returned 0.

This will help when raising the maximum allowed channel count in mpv.
Some code in af_lavrresample relies on it, more or less.

One change is that unknown channel maps won't result in lavc standard
channel layouts anymore, just a set of random speakers. This should be
fine, as the caller of mp_chmap_to_lavc_unchecked() should handle these
cases. For mp_chmap_reorder_to_lavc() this is not so clear anymore, but
should also be ok.

For normal channel maps, simply dropping NA channels is still the
correct and wanted behavior.

Currently, the mpv maximum channel count is 8. This commit is
preparation for raising this limit.

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