Add private API to GDK to move these variables from the environment into
static scope. Also move the DESKTOP_STARTUP_ID validation here to reduce
code duplication.

Use constructors to read them as early as possible; however, do not
unset them until first requested. This avoids breaking gnome-shell and
gnome-settings-daemon, which want to use the DESKTOP_AUTOSTART_ID in
their own gnome-session clients.

Fixes GNOME/gtk#1761