Latest version of xray-core made a change to support FullCone NAT,
which would break UDP connection from v2ray-core backend server.
So added the option for v2ray-core users, to make sure UDP works
as expected.

Signed-off-by: Tianling Shen <cnsztl@project-openwrt.eu.org>

Fix starting problem:
Starting function should be named 'start_service' instead of 'start_instance'.

Fix reloading problem:
Register reload tigger for uci config itself.
And, xray does not support reload currently, so use legacy restart as reload.

Fixes: 6c9b96352f ("xray-core: add init script")

Signed-off-by: Tianling Shen <cnsztl@project-openwrt.eu.org>

Add init script so the users could run it with their configs quickly.

Signed-off-by: Tianling Shen <cnsztl@project-openwrt.eu.org>