main: defer startup until we know our session mode

commit 92083eaf made
session mode loading an asynchronous operation.

Aspects of the session mode aren't known immediately at
start up. For instance, sessionMode.isGreeter returns
false for greeter sessions until the asynchronous
operation completes.

This commit defers start up processing until the session
mode is fully known.

https://bugzilla.gnome.org/show_bug.cgi?id=682429