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/provider/vyprvpn/openvpnconf.go

@@ -3,20 +3,21 @@ package vyprvpn
 import (
 	"strconv"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
 	"github.com/qdm12/gluetun/internal/provider/utils"
 )
 
 func (v *Vyprvpn) BuildConf(connection models.Connection,
-	settings configuration.OpenVPN) (lines []string, err error) {
+	settings settings.OpenVPN) (lines []string, err error) {
 	if len(settings.Ciphers) == 0 {
 		settings.Ciphers = []string{constants.AES256cbc}
 	}
 
-	if settings.Auth == "" {
-		settings.Auth = constants.SHA256
+	auth := *settings.Auth
+	if auth == "" {
+		auth = constants.SHA256
 	}
 
 	lines = []string{
@@ -24,7 +25,7 @@ func (v *Vyprvpn) BuildConf(connection models.Connection,
 		"nobind",
 		"tls-exit",
 		"dev " + settings.Interface,
-		"verb " + strconv.Itoa(settings.Verbosity),
+		"verb " + strconv.Itoa(*settings.Verbosity),
 
 		// Vyprvpn specific
 		"ping 10",
@@ -32,7 +33,7 @@ func (v *Vyprvpn) BuildConf(connection models.Connection,
 		// "verify-x509-name lu1.vyprvpn.com name",
 		"tls-cipher TLS-ECDHE-RSA-WITH-AES-256-GCM-SHA384:TLS-DHE-RSA-WITH-AES-256-CBC-SHA256:TLS-DHE-RSA-WITH-AES-256-CBC-SHA", //nolint:lll
 		"auth-user-pass " + constants.OpenVPNAuthConf,
-		"auth " + settings.Auth,
+		"auth " + auth,
 		"comp-lzo",
 
 		// Added constant values
@@ -53,14 +54,14 @@ func (v *Vyprvpn) BuildConf(connection models.Connection,
 		lines = append(lines, "explicit-exit-notify")
 	}
 
-	if !settings.Root {
+	if !*settings.Root {
 		lines = append(lines, "user "+settings.ProcUser)
 		lines = append(lines, "persist-tun")
 		lines = append(lines, "persist-key")
 	}
 
-	if settings.MSSFix > 0 {
-		lines = append(lines, "mssfix "+strconv.Itoa(int(settings.MSSFix)))
+	if *settings.MSSFix > 0 {
+		lines = append(lines, "mssfix "+strconv.Itoa(int(*settings.MSSFix)))
 	}
 
 	lines = append(lines, utils.WrapOpenvpnCA(