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/server.go

@@ -0,0 +1,71 @@
+package settings
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gotree"
+)
+
+// ControlServer contains settings to customize the control server operation.
+type ControlServer struct {
+	// Port is the listening port to use.
+	// It can be set to 0 to bind to a random port.
+	// It cannot be nil in the internal state.
+	// TODO change to address
+	Port *uint16
+	// Log can be true or false to enable logging on requests.
+	// It cannot be nil in the internal state.
+	Log *bool
+}
+
+func (c ControlServer) validate() (err error) {
+	uid := os.Getuid()
+	const maxPrivilegedPort uint16 = 1023
+	if uid != 0 && *c.Port <= maxPrivilegedPort {
+		return fmt.Errorf("%w: %d when running with user ID %d",
+			ErrControlServerPrivilegedPort, *c.Port, uid)
+	}
+
+	return nil
+}
+
+func (c *ControlServer) copy() (copied ControlServer) {
+	return ControlServer{
+		Port: helpers.CopyUint16Ptr(c.Port),
+		Log:  helpers.CopyBoolPtr(c.Log),
+	}
+}
+
+// mergeWith merges the other settings into any
+// unset field of the receiver settings object.
+func (c *ControlServer) mergeWith(other ControlServer) {
+	c.Port = helpers.MergeWithUint16(c.Port, other.Port)
+	c.Log = helpers.MergeWithBool(c.Log, other.Log)
+}
+
+// overrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (c *ControlServer) overrideWith(other ControlServer) {
+	c.Port = helpers.MergeWithUint16(c.Port, other.Port)
+	c.Log = helpers.MergeWithBool(c.Log, other.Log)
+}
+
+func (c *ControlServer) setDefaults() {
+	const defaultPort = 8000
+	c.Port = helpers.DefaultUint16(c.Port, defaultPort)
+	c.Log = helpers.DefaultBool(c.Log, true)
+}
+
+func (c ControlServer) String() string {
+	return c.toLinesNode().String()
+}
+
+func (c ControlServer) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Control server settings:")
+	node.Appendf("Listening port: %d", *c.Port)
+	node.Appendf("Logging: %s", helpers.BoolPtrToYesNo(c.Log))
+	return node
+}