VPNSP value custom for OpenVPN custom config files (#621)

- Retro-compatibility: `OPENVPN_CUSTOM_CONFIG` set implies `VPNSP=custom`
- Change: `up` and `down` options are not filtered out
- Change: `OPENVPN_INTERFACE` overrides the network interface defined in the configuration file
- Change: `PORT` overrides any port found in the configuration file
- Feat: config file is read when building the OpenVPN configuration, so it's effectively reloaded on VPN restarts
- Feat: extract values from custom file at start to log out valid settings
- Maint: `internal/openvpn/extract` package instead of `internal/openvpn/custom` package
- Maint: All providers' `BuildConf` method return an error
- Maint: rename `CustomConfig` to `ConfFile` in Settings structures

internal/cli/openvpnconfig.go

@@ -34,7 +34,11 @@ func (c *CLI) OpenvpnConfig(logger logging.Logger, env params.Interface) error {
 	if err != nil {
 		return err
 	}
-	lines := providerConf.BuildConf(connection, allSettings.VPN.OpenVPN)
+	lines, err := providerConf.BuildConf(connection, allSettings.VPN.OpenVPN)
+	if err != nil {
+		return err
+	}
+
 	fmt.Println(strings.Join(lines, "\n"))
 	return nil
 }