wgengine/magicsock: improve don't fragment bit set/get support

Add an enable/disable argument to setDontFragment() in preparation for dynamic
enable/disable of peer path MTU discovery. Add getDontFragment() to get the
status of the don't fragment bit from a socket.

Updates #311

Co-authored-by: James Tucker <james@tailscale.com>
Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/peermtu_unix.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (darwin && !ios) || (linux && !android)
+
+package magicsock
+
+import (
+	"syscall"
+)
+
+// getIPProto returns the value of the get/setsockopt proto argument necessary
+// to set an IP sockopt that corresponds with the string network, which must be
+// "udp4" or "udp6".
+func getIPProto(network string) int {
+	if network == "udp4" {
+		return syscall.IPPROTO_IP
+	}
+	return syscall.IPPROTO_IPV6
+}
+
+// connControl allows the caller to run a system call on the socket underlying
+// Conn specified by the string network, which must be "udp4" or "udp6". If the
+// pconn type implements the syscall method, this function returns the value of
+// of the system call fn called with the fd of the socket as its arg (or the
+// error from rc.Control() if that fails). Otherwise it returns the error
+// errUnsupportedConnType.
+func (c *Conn) connControl(network string, fn func(fd uintptr)) error {
+	pconn := c.pconn4.pconn
+	if network == "udp6" {
+		pconn = c.pconn6.pconn
+	}
+	sc, ok := pconn.(syscall.Conn)
+	if !ok {
+		return errUnsupportedConnType
+	}
+	rc, err := sc.SyscallConn()
+	if err != nil {
+		return err
+	}
+	return rc.Control(fn)
+}
+
+func CanPMTUD() bool {
+	return debugEnablePMTUD()
+}