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/shadowsocks.go

@@ -0,0 +1,68 @@
+package settings
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gotree"
+	"github.com/qdm12/ss-server/pkg/tcpudp"
+)
+
+// Shadowsocks contains settings to configure the Shadowsocks server.
+type Shadowsocks struct {
+	// Enabled is true if the server should be running.
+	// It defaults to false, and cannot be nil in the internal state.
+	Enabled *bool
+	// Settings are settings for the TCP+UDP server.
+	tcpudp.Settings
+}
+
+func (s Shadowsocks) validate() (err error) {
+	return s.Settings.Validate()
+}
+
+func (s *Shadowsocks) copy() (copied Shadowsocks) {
+	return Shadowsocks{
+		Enabled:  helpers.CopyBoolPtr(s.Enabled),
+		Settings: s.Settings.Copy(),
+	}
+}
+
+// mergeWith merges the other settings into any
+// unset field of the receiver settings object.
+func (s *Shadowsocks) mergeWith(other Shadowsocks) {
+	s.Enabled = helpers.MergeWithBool(s.Enabled, other.Enabled)
+	s.Settings.MergeWith(other.Settings)
+}
+
+// overrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (s *Shadowsocks) overrideWith(other Shadowsocks) {
+	s.Enabled = helpers.OverrideWithBool(s.Enabled, other.Enabled)
+	s.Settings.OverrideWith(other.Settings)
+}
+
+func (s *Shadowsocks) setDefaults() {
+	s.Enabled = helpers.DefaultBool(s.Enabled, false)
+	s.Settings.SetDefaults()
+}
+
+func (s Shadowsocks) String() string {
+	return s.toLinesNode().String()
+}
+
+func (s Shadowsocks) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Shadowsocks server settings:")
+
+	node.Appendf("Enabled: %s", helpers.BoolPtrToYesNo(s.Enabled))
+	if !*s.Enabled {
+		return node
+	}
+
+	// TODO have ToLinesNode in qdm12/ss-server
+	node.Appendf("Listening address: %s", s.Address)
+	node.Appendf("Cipher: %s", s.CipherName)
+	node.Appendf("Password: %s", helpers.ObfuscatePassword(*s.Password))
+	node.Appendf("Log addresses: %s", helpers.BoolPtrToYesNo(s.LogAddresses))
+
+	return node
+}