This commit adds reauthentication support for
screensavers and user switching to use.

1) It adds a "verification mode" argument to the GdmSession constructor
that tweaks the behavior of how the session worker acts to fit login or
unlock scenarios better.

2) It adds a way for programs to open a communication channel for user
verification to already runnings sessions (so reauthentication happens
in the context of the session).