[th/nm-sudo-exit-on-idle-race] fix race condition during exit-on-idle for nm-sudo D-Bus service

Getting exit-on-idle right is not entirely trivial. Luckily, the main-loop as nm-sudo runs it, makes this trivial to integrate.

See https://lists.freedesktop.org/archives/dbus/2015-May/016671.html and the linked sources:

• https://bugs.freedesktop.org/show_bug.cgi?id=11454
• https://github.com/cgwalters/test-exit-on-idle

Note that this race also affects NetworkManager-dispatcher!