Reload, don't restart, dbus.service

Many bus clients get hopelessly confused when dbus-daemon is
restarted. So let's not do that.

Of course, this is not ideal either, because we end up stuck with a
possibly outdated dbus-daemon. But that issue will become irrelevant
in the glorious kdbus-based future.

Hopefully this also gets rid of systemd getting stuck after
dbus-daemon is restarted:

Apr 01 15:37:50 mandark systemd[1]: Failed to register match for Disconnected message: Connection timed out
Apr 01 15:37:50 mandark systemd[1]: Looping too fast. Throttling execution a little.
Apr 01 15:37:51 mandark systemd[1]: Looping too fast. Throttling execution a little.
...

nixos/modules/services/system/dbus.nix

@@ -130,6 +130,9 @@ in
         config.system.path
       ];
 
+    # Don't restart dbus-daemon. Bad things tend to happen if we do.
+    systemd.services.dbus.reloadIfChanged = true;
+
     environment.pathsToLink = [ "/etc/dbus-1" "/share/dbus-1" ];
 
   };