This commit adds a number of fixes to the OpenVPN up/down hotplug command
wrapper which currently fails to actually invoke user defined up and down
commands for uci configurations not using external native configurations.

 - Use the `--setenv` to pass the user configured `up` and `down` commands
   as `user_up` and `user_down` environment variables respectively

 - Instead of attempting to scrape the `up` and `down` settings from the
   (possibly generated) native OpenVPN configuration in
   `/etc/hotplug.d/openvpn/01-user`, read them from the respective
   environment variables instead

 - Fix parsing of native configuration values in `get_openvpn_option()`;
   first try to parse a given setting as single quoted value, then as
   double quoted and finally as non-quoted, potentially white-space
   escaped one. This ensures that `up '/bin/foo'` is interpreted as
   `/bin/foo` and not `'/bin/foo'`

Ref: https://forum.openwrt.org/t/openvpn-up-down-configuration-ignored/91126
Supersedes: #15121, #15284
Signed-off-by: Jo-Philipp Wich <jo@mein.io>