bug#38172: fixing dangerous PulseAudio defaults and giving it a record type

[raingloom]

Discussion from IRC:
http://logs.guix.gnu.org/guix/2019-11-11.log#213424

Basically it makes volume settings behave in erratic and surprising
ways that are not obvious even to seasoned Linux users, including
sudden loud noises.

The fix is as simple as adding "flat-volumes = no"
to .config/pulse/daemon.conf, but this should really be a system
default.

However, rather than patching it, the consensus seems to be that we
should create a <pulseaudio-configuration> record type.
http://logs.guix.gnu.org/guix/2019-11-11.log#213953