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/settings/log.go

@@ -0,0 +1,51 @@
+package settings
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/golibs/logging"
+	"github.com/qdm12/gotree"
+)
+
+// Log contains settings to configure the logger.
+type Log struct {
+	// Level is the log level of the logger.
+	// It cannot be nil in the internal state.
+	Level *logging.Level
+}
+
+func (l Log) validate() (err error) {
+	return nil
+}
+
+func (l *Log) copy() (copied Log) {
+	return Log{
+		Level: helpers.CopyLogLevelPtr(l.Level),
+	}
+}
+
+// mergeWith merges the other settings into any
+// unset field of the receiver settings object.
+func (l *Log) mergeWith(other Log) {
+	l.Level = helpers.MergeWithLogLevel(l.Level, other.Level)
+}
+
+// overrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (l *Log) overrideWith(other Log) {
+	l.Level = helpers.OverrideWithLogLevel(l.Level, other.Level)
+}
+
+func (l *Log) setDefaults() {
+	l.Level = helpers.DefaultLogLevel(l.Level, logging.LevelInfo)
+}
+
+func (l Log) String() string {
+	return l.toLinesNode().String()
+}
+
+func (l Log) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Log settings:")
+	node.Appendf("Log level: %s", l.Level.String())
+	return node
+}