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

[Leo Prikler]

Being the one who suggested record types, I came up with a quick and
dirty solution.  I have no idea, whether this works yet -- I only ran
guix system build to check whether files are actually put in /etc.  I
don't know, whether PulseAudio will actually read this file or rather
use the one in its own package.

This solution mimics ALSA's "extra-options" field, which allows direct
writing of ALSA "options".  I've decided, that strings best be
concatenated with newlines, and lists with two entries are key-value
pairs.  This structure allows for an easy writing of the two most used
constructs in such files; free-form comments and assignments.

The above applies to both client.conf and daemon.conf.  Ideally, I'd
like to replace them with their own records, but that's for another
day.  default.pa and system.pa are actually scripts, so I just use
mixed-text-file to concatenate user input.   Since an empty script is
probably not useful, they're symlinked to the package files by default.

Let me know if that patch works for you.  Also let me know if my design
choices are weird to you, I'm open to other approaches.

Regards,

Leo

0001-services-Add-PulseAudio-service.patch
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