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_stubs.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (!linux && !darwin) || android || ios
+
+package magicsock
+
+import (
+	"errors"
+
+	"tailscale.com/types/nettype"
+)
+
+// setDontFragment sets the dontfragment sockopt on pconn on the platforms that support it,
+// for both IPv4 and IPv6.
+// (C.f. https://datatracker.ietf.org/doc/html/rfc3542#section-11.2 for IPv6 fragmentation)
+func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
+	return errors.New("setting don't fragment bit not supported on this OS")
+}
+
+// CanPMTUD returns whether this platform supports performing peet path MTU discovery.
+func CanPMTUD() bool {
+	return false
+}