Add timeout for udev rule

The version of udev in wheezy kills rules that take longer than 10s. The
long timeout of 120s is to be on the safe side.

Rule processing in udev is synchronous. So no other rules are processed
while this rule is executing. This is intentional, live-guest could have
some race conditions.
Adaptions for gdm3

GDM was replaced with GDM3 in Debian wheezy. GDM3's greeter opens it's
own console-kit session and utmp is only updated after the pam-script
hook is executed.
Add "(Guest)" to GECOS field for live-guest user

GDM3 only shows the GECOS field and not the UNIX user name in the
greeter. The user name is only shown in a tooltip. Add "(Guest)" to
easier distinguish users with the same GECOS value.
Don't fail if a group does not exist

usermod -G fails if one of the listed groups does not exist. Add the
user to the groups individually. This is the same method which is used
by user-setup.
Update to Debian Policy 3.9.4, no changes needed
Adapt MOUNT_BASE_DIR for wheezy

The live-boot package in wheezy mounts all live related filesystems in
/lib/live/mount instead of just /live. Adapt live-guest to this.
Release version 0.18