objectManager: clear inhibitor on unregistered interfaces

A D-Bus service can export more supported interfaces than the
shell cares about.  In those cases, we avoid creating proxies,
but neglect to finish things up so the object manager class
knows it can mark itself loaded.

This commit makes sure we do the proper finishing, so the object
manager still loads in the face of unsupported interfaces.

https://bugzilla.gnome.org/show_bug.cgi?id=706542
objectManager: fix indentation
gdmUtil: make _startService support no username

commit fd11ad95f61b904d6f7014f8d56c1c27d88af65c factored
out duplicated code, but unintentionally dropped support
for beginning verification without a username.

This commit brings it back.

https://bugzilla.gnome.org/show_bug.cgi?id=706542