Prevent selection of too few DCP channels (#611).

Prevent selection of too few DCP channels (#611).

This makes Film::audio_channels() return a minimum of any active
processor's output channel count, even if fewer DCP channels have
been selected.  It may have been neater code-wise to make Player cope
if such a setting is made, but it would probably confuse people
if we don't auto-fix it (like this commit does).