Fix awacs(4) audio on some iMac G3 models

Fix awacs(4) audio on some iMac G3 models

This code affects iMac G3s with a slot-loading CD drive (from late
1999 to 2001, after the older tray-loading iMac G3s, and before the
iMac G4s).  Detect such iMacs by their model string (PowerMac2,1 or
PowerMac2,2 or PowerMac4,1), and configure their headphones and
internal speakers in a different way.  These iMacs have 3 places to
connect headphones (2 on front, 1 on right side).  They also have no
audio connection to the CD drive, so turn off the noisy CD input.

From evan (dot) jss (at) protonmail (dot) ch; I edited the code to get
the model string.