Deprecate using strings for these fields and prefer user-account
(resp. user-group) instead to avoid duplication within account-service-type.
Fixes#61570 <https://issues.guix.gnu.org/61570>.
* gnu/services/audio.scm (%mpd-user, %mpd-group)
(mpd-serialize-user-account, mpd-serialize-user-group)
(mpd-user-sanitizer, mpd-group-sanitizer): New variables.
(mpd-configuration)[user]: Use user-account as value type.
Sanitize via mpd-user-sanitizer.
[group]: Use user-group as value type.
Sanitize via mpd-group-sanitizer.
(mpd-shepherd-service): Adjust accordingly.
(mpd-accounts): Likewise.
* doc/guix.texi (Audio Services)[Music Player Daemon]: Likewise.
Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>
865df7f080