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/batching_conn.go

@@ -9,6 +9,7 @@ import (
 	"net/netip"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"time"
 
 	"golang.org/x/net/ipv6"
@@ -192,3 +193,11 @@ retry:
 	}
 	return err
 }
+
+func (c *batchingUDPConn) SyscallConn() (syscall.RawConn, error) {
+	sc, ok := c.pc.(syscall.Conn)
+	if !ok {
+		return nil, errUnsupportedConnType
+	}
+	return sc.SyscallConn()
+}