Named channel configurations

[olofson]

It would be useful to be able to define named channel configurations, beyond just specifying a number of channels with implicit assignments. One channel is mono, two channels is stereo and... then what? And we might want two channels to mean "mono + effect send" in some cases!

This might be useful in voice structure definitions (say "stereo" instead of "2" for unit input and/or output count), but more importantly, it is pretty much required for sorting out the current situation with channel count mismatches in parent/child voice relations.

As it is, all we could do is hardwire some default mapping behavior when spawning a program that refuses to match the parent voice channel count, and for anything beyond mono and stereo, that's not always going to give the intended result.

Something like this:

cconfig mono {
    center
}

cconfig stereo {
    left
    right
}

cconfig sr71 {
    frontl; center; frontr
    sidel;          sider
    rearl;          rearr
            sub
}

cconfig synthbus {
    left; right
    reverb
    chorus
}

(Somewhat related to #13 and #14.)