Introduce a new annotation flag "org.freedesktop.policykit.exec.allow_gui"
which will cause pkexec to preserve $DISPLAY and $XAUTHORITY. With this, the
remaining few legacy X11 programs which still need to run as root can finally
be migrated away from gksu (or similar) to pkexec, with the help of some
.polkit files. This will provide a consistent UI and also help with making the
authentication dialogs less spoofable.

Relax validate_environment_variable() to allow '/' in $XAUTHORITY, as this
variable actually is a full path.

Signed-off-by: David Zeuthen <davidz@redhat.com>