luci-base: add transitional /etc/init.d/ucitrack

Ship an /etc/init.d/ucitrack for spawning a virtual service with the sole
purpose to track the configurations and dependencies formerly handled by
luci-reload.

Once all LuCI supported services ship with procd compatible init scripts,
the uci track support can be dropped.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>