wgengine/magicsock: rename files for peer MTU

Rename dontfrag* to peermtu* to prepare for more peer MTU related code going
into these files.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/peermtu_linux.go

@@ -0,0 +1,34 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && !android
+
+package magicsock
+
+import (
+	"net"
+	"syscall"
+
+	"tailscale.com/types/nettype"
+)
+
+func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
+	if c, ok := pconn.(*net.UDPConn); ok {
+		rc, err := c.SyscallConn()
+		if err == nil {
+			rc.Control(func(fd uintptr) {
+				if network == "udp4" {
+					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
+				}
+				if network == "udp6" {
+					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
+				}
+			})
+		}
+	}
+	return err
+}
+
+func CanPMTUD() bool {
+	return debugPMTUD() // only if the envknob is enabled, for now.
+}