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>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38632 3c298f89-4303-0410-b956-a3cf2f4a3e73