UDisks dbus async calls don't work sometimes when a second mainloop exists

[Havoc Pennington]

The issue may be that the dbus-glib mainloop source is set to
nonrecursive. This is because historically the thread lock on
DBusConnection was not recursive so you would deadlock if you tried to
dispatch DBusConnection from within an existing dispatch.

I'm not sure whether the main loop source is still nonrecursive or
whether it needs to be. It may be a relic.

Havoc