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/openvpn/extract/extractor.go

@@ -0,0 +1,18 @@
+package extract
+
+import (
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+var _ Interface = (*Extractor)(nil)
+
+type Interface interface {
+	Data(filepath string) (lines []string,
+		connection models.Connection, err error)
+}
+
+type Extractor struct{}
+
+func New() *Extractor {
+	return new(Extractor)
+}