Add support for inhibiting automount

When connecting to virtual machines with usb-device redirection, such as Spice
enabled vms, automount may get in the way. Specifically if auto-usbredir is
enabled in the vm-viewer, then the usbredir code and the automount code race
for who gets to the device first.

If the automount code wins the race this is a problem, since usbredir causes a
device-disconnect (iow the usb mass storage driver sees an unplug), so in the
end usbredir always wins, and we end up with a non clean potentially corrupt
filesystem. Also see:
https://bugzilla.redhat.com/show_bug.cgi?id=812972



There for the need exists to be able to inhibit gnome-shell's automounting,
since all other inhibits run through gnome-session, I've chosen to do the same
for the automount-inhibiting. I've also submitted a patch to gnome-session to
reserve flag value 16 for this, see bug 678595.

This patch adds support to gnome-shell to honor this new inhibit flag.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

https://bugzilla.gnome.org/show_bug.cgi?id=678597