Set the systemd user service to use "mixed" killmode, which lets waybar
stop its module scripts. This fixes issues where waybar blocks shutdown
until systemd sends a SIGKILL to waybar child processes.
The current definition makes waybar wait for dbus.service, but that
never happens because dbus.service is started on demand by
dbus.socket.
Per systemd docs:
https://www.freedesktop.org/software/systemd/man/systemd.service.html#Implicit%20Dependencies
- Services with Type=dbus set automatically acquire dependencies of
type Requires= and After= on dbus.socket.
- Socket activated services are automatically ordered after their
activating .socket units via an automatic After= dependency.
Services also pull in all .socket units listed in Sockets= via
automatic Wants= and After= dependencies.
Removing Requisite/After makes the service properly start for me,
simply specifying Type=dbus is enough.
See #1370
