disable automounting while screen is locked

On the recent Shmoocon there was a presentation by Jon Larimer demonstrating
how to abuse vulnerabilities and bugs, or even just creating socially or
security compromising thumbnails in mounting and thumbnailing, which happens o
automounting USB drives. This is a particular issue when this happens on a
locked box where the attacker doesn't otherwise have access to the user
account:

  http://www.net-security.org/secworld.php?id=10544

Disable automounting if the GNOME screen saver is currently locked.

Backported from gnome-settings-daemon trunk commits 71deedf7 and 90c0f8676.

https://bugzilla.gnome.org/show_bug.cgi?id=642020