Save/restore default audio control settings
PSARC/2010/116 added persistent per-user volume settings (only for a single device) on GDM session login/logout.
It may be useful to use
audioctl load-controls to load default audio control settings systemwide on boot as part of
svc:/system/device/audio:default in cases where an input or output has to be muted or a mixer has an inappropriate default volume (driver or hardware issue).
We may also allow automatic saving which will mean the last user-specified volume is restored -- unclear if this is always desirable on a multi-user system. Otherwise
audioctl save-controls will have to be manually run by an administrator.
Or a compromise could be that we autosave unless a specific file already exists.