wgengine,net,ipn,disco: split up and define different types of MTU

Prepare for path MTU discovery by splitting up the concept of
DefaultMTU() into the concepts of the Tailscale TUN MTU, MTUs of
underlying network interfaces, minimum "safe" TUN MTU, user configured
TUN MTU, probed path MTU to a peer, and maximum probed MTU. Add a set
of likely MTUs to probe.

Updates #311

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

ipn/localapi/localapi.go

@@ -36,7 +36,6 @@ import (
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/netutil"
 	"tailscale.com/net/portmapper"
-	"tailscale.com/net/tstun"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
 	"tailscale.com/tstime"
@@ -51,6 +50,7 @@ import (
 	"tailscale.com/util/osdiag"
 	"tailscale.com/util/rands"
 	"tailscale.com/version"
+	"tailscale.com/wgengine/magicsock"
 )
 
 type localAPIHandler func(*Handler, http.ResponseWriter, *http.Request)
@@ -1382,8 +1382,8 @@ func (h *Handler) servePing(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "'size' parameter is only supported with disco pings", 400)
 			return
 		}
-		if size > int(tstun.DefaultMTU()) {
-			http.Error(w, fmt.Sprintf("maximum value for 'size' is %v", tstun.DefaultMTU()), 400)
+		if size > magicsock.MaxDiscoPingSize {
+			http.Error(w, fmt.Sprintf("maximum value for 'size' is %v", magicsock.MaxDiscoPingSize), 400)
 			return
 		}
 	}