pipewire: rework PACKAGECONFIG for systemd service files

pipewire project have config option to enable installation of systemd
user and/or system services. Before update of pipewire to current version
(02de7746) only system service files was added, after update both user
and system service files was added. This commit will make it possible to
configure which service files that is wanted. Default PACKAGECONFIG is
to add systemd service files as it was before version upgrade.

Yocto systemd class currently does not handle user service files during package
and those files are only installed in rootfs but user service is not enabled as a
system service is.

Another use case is where you want your own service files and can disable the
default ones from pipewire and add your own in a bbappend.

Signed-off-by: Peter Bergin <peter@berginkonsult.se>