openvpn: add generic hotplug mechanism

Pass a default --up and --down executable to each started OpenVPN instance
which triggers /etc/hotplug.d/openvpn/ scripts whenever an instance
goes up or down.

User-configured up and down scripts are invoked by the default shipped
01-user hotplug handler to ensure that existing setups continue to work
as before.

As a consequence of this change, the up, down and script_security OpenVPN
options are removed from the option file, since we're always passing them
via the command line, they do not need to get included into the generated
configuration.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
[reword commit message, move hotplug executable to /usr/libexec]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/network/services/openvpn/files/usr/libexec/openvpn-hotplug

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+ACTION=$1
+shift
+INSTANCE=$1
+shift
+
+export ACTION=$ACTION
+export INSTANCE=$INSTANCE
+exec /sbin/hotplug-call openvpn "$@"