openvpn: clean up and fix the init script

openvpn: clean up and fix the init script

- clean up duplication of procd instance handling code
- using --cd *after* --config is rather pointless
- to be able to log errors properly, --syslog needs to be passed before --config
- tell procd about the generated or referenced config file instead of
  the uci file. this avoids having to restart all instances if only one
  of them changes.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
SVN-Revision: 38632