net: add mptcpStatus type

This new type will be used in the following commits.

The goal is to have a tristate, an enum with three values:

- system default (0)
- enabled
- disabled

The system default value is linked to defaultMPTCPEnabled: disabled by
default for the moment. Users will be able to force enabling/disabling
MPTCP or use the default behaviour.

This work has been co-developped by Gregory Detal
<gregory.detal@tessares.net>.

Updates #56539

Change-Id: I8fa0cad7a18ca967508799fc828ef060b27683d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/477735
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/dial.go

@@ -11,12 +11,45 @@ import (
 	"time"
 )
 
-// defaultTCPKeepAlive is a default constant value for TCPKeepAlive times
-// See golang.org/issue/31510
 const (
+	// defaultTCPKeepAlive is a default constant value for TCPKeepAlive times
+	// See go.dev/issue/31510
 	defaultTCPKeepAlive = 15 * time.Second
+
+	// For the moment, MultiPath TCP is not used by default
+	// See go.dev/issue/56539
+	defaultMPTCPEnabled = false
 )
 
+// mptcpStatus is a tristate for Multipath TCP, see go.dev/issue/56539
+type mptcpStatus uint8
+
+const (
+	// The value 0 is the system default, linked to defaultMPTCPEnabled
+	mptcpUseDefault mptcpStatus = iota
+	mptcpEnabled
+	mptcpDisabled
+)
+
+func (m *mptcpStatus) get() bool {
+	switch *m {
+	case mptcpEnabled:
+		return true
+	case mptcpDisabled:
+		return false
+	}
+
+	return defaultMPTCPEnabled
+}
+
+func (m *mptcpStatus) set(use bool) {
+	if use {
+		*m = mptcpEnabled
+	} else {
+		*m = mptcpDisabled
+	}
+}
+
 // A Dialer contains options for connecting to an address.
 //
 // The zero value for each field is equivalent to dialing