add a systemd user service for each D-Bus session service

[Simon McVittie]

Please consider adding a systemd user service for each D-Bus session service. This means that, under a systemd user session where dbus-daemon uses systemd for activation, they will correctly go into their own cgroups instead of being treated as part of dbus.service.

The patch I'm attaching is a prerequisite for that: I'll attach the actual fix in a moment. Description for the first patch:

"""
Give the D-Bus service files the correct names

Best-practice is that if you implement a session service whose
well-known name (Name field) is com.example.MyService1, and it
should be service-activatable, then it should install
${datadir}/dbus-1/services/com.example.MyService1.service.
This ensures that two non-conflicting packages cannot provide the
same service name with the same search-path priority (i.e. in the
same directory).

If there are two session services with the same Name in the same
directory, dbus-daemon will arbitrarily choose one of them, which
is unlikely to be the desired result.
"""

[Simon McVittie (smcv)]

This means that, under a systemd user session where dbus-daemon uses
systemd for activation, they will correctly go into their own cgroups
instead of being treated as part of dbus.service.