Openvpn: add missing script event options

Openvpn: add missing script event options

Maintainer: @mkrkn  @neheb
Compile tested: aarch64, cortex-a53, OpenWRT Master
Run tested: Dynalink DL-WRX36

Description:
[A previous commit](https://github.com/openwrt/packages/commit/f8a8b71e26b9bdbf86fbb7d4d1482637af7f3ba4) has added more script event options.
However it looked like that commit was not complete as it stops the use of the script events route-up, route-pre-down, and ipchange when those are placed in the openvpn config file.

This PR fixes a regression that makes it problematic to specify certain event options in the OpenVPN configuration file.

Discussion in [this thread](https://forum.openwrt.org/t/openvpn-custom-route-up-script-in-23-05-rc2/167105/13) and [here](https://forum.openwrt.org/t/openvpn-route-up-and-route-pre-down-broken-in-23-05/176568)

Please have a look and consider implementing or make it possible to use all script event options in the openvpn config file in another way.

Pull request has been discussed and improved with the help of @AuthorReflex, see: https://github.com/openwrt/packages/pull/21732

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