openvpn: autostart openvpn instances for each .conf file in /etc/openvpn

Align init behaviour with other distros by starting an OpenVPN instance
for each config file found in /etc/openvpn/. This removes the additional
requirement to "register" the configs with uci and thus simplifies the
setup.

Make sure to respect the disabled state in uci to not suddenly autostart
instances which have been previously set to disabled, also skip configs
which are already started due to uci configuration.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44310