chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/sources/env/openvpnselection.go

@@ -0,0 +1,78 @@
+package env
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/govalid/port"
+)
+
+func (r *Reader) readOpenVPNSelection() (
+	selection settings.OpenVPNSelection, err error) {
+	selection.TCP, err = r.readOpenVPNProtocol()
+	if err != nil {
+		return selection, err
+	}
+
+	selection.CustomPort, err = r.readOpenVPNCustomPort()
+	if err != nil {
+		return selection, err
+	}
+
+	selection.PIAEncPreset = r.readPIAEncryptionPreset()
+
+	return selection, nil
+}
+
+var ErrOpenVPNProtocolNotValid = errors.New("OpenVPN protocol is not valid")
+
+func (r *Reader) readOpenVPNProtocol() (tcp *bool, err error) {
+	envKey := "OPENVPN_PROTOCOL"
+	protocol := strings.ToLower(os.Getenv("OPENVPN_PROTOCOL"))
+	if protocol == "" {
+		// Retro-compatibility
+		protocol = strings.ToLower(os.Getenv("PROTOCOL"))
+		if protocol != "" {
+			envKey = "PROTOCOL"
+			r.onRetroActive("PROTOCOL", "OPENVPN_PROTOCOL")
+		}
+	}
+
+	switch protocol {
+	case "":
+		return nil, nil //nolint:nilnil
+	case constants.UDP:
+		return boolPtr(false), nil
+	case constants.TCP:
+		return boolPtr(true), nil
+	default:
+		return nil, fmt.Errorf("environment variable %s: %w: %s",
+			envKey, ErrOpenVPNProtocolNotValid, protocol)
+	}
+}
+
+func (r *Reader) readOpenVPNCustomPort() (customPort *uint16, err error) {
+	key := "OPENVPN_PORT"
+	s := os.Getenv(key)
+	if s == "" {
+		// Retro-compatibility
+		key = "PORT"
+		s = os.Getenv(key)
+		if s == "" {
+			return nil, nil //nolint:nilnil
+		}
+		r.onRetroActive("PORT", "OPENVPN_PORT")
+	}
+
+	customPort = new(uint16)
+	*customPort, err = port.Validate(s)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable %s: %w", key, err)
+	}
+
+	return customPort, nil
+}