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/provider/nordvpn/openvpnconf.go

@@ -10,7 +10,7 @@ import (
 )
 
 func (n *Nordvpn) BuildConf(connection models.Connection,
-	settings configuration.OpenVPN) (lines []string) {
+	settings configuration.OpenVPN) (lines []string, err error) {
 	if settings.Cipher == "" {
 		settings.Cipher = constants.AES256cbc
 	}
@@ -81,5 +81,5 @@ func (n *Nordvpn) BuildConf(connection models.Connection,
 
 	lines = append(lines, "")
 
-	return lines
+	return lines, nil
 }