openvpn: add missing script-security

openvpn: add missing script-security

Maintainer: @mkrkn  @neheb

Compile tested: armv7, cortexA15, OpenWRT 23.05
Run tested: Linksys EA8500

Compile tested: armv8, cortexA53, OpenWRT main
Run tested: Dynalink DL-WRX36

Description:
Script-security is always 2 and cannot be changed from the openvpn config file due to a missing rule in openvpn.init.

This is discussed in issue #23014

This patch adds the missing rule in openvpn.init to parse script-security from the openvpn config file.

Signed-off-by: Erik Conijn <egc112@msn.com>