config_mixer: rework volume handling, get rid of aumix and fix_ibm_sound()

config_mixer of /etc/grml/autoconfig.functions fails on quite a
lot of IBM systems with "amixer: Unable to find simple control
'PCM',0", so the mixer volumes aren't adjusted as wanted.

This update gets rid of aumix, doesn't use  fix_ibm_sound() any
longer and also addresses systems with several soundcard devices.

Closes: issue850