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>

wgengine/magicsock/endpoint.go

@@ -422,6 +422,10 @@ func (de *endpoint) noteActiveLocked() {
 	}
 }
 
+// MaxDiscoPingSize is the largest useful ping message size that we
+// can send - the maximum packet size minus the IPv4 and UDP headers.
+var MaxDiscoPingSize = tstun.MaxPacketSize - 20 - 8
+
 // cliPing starts a ping for the "tailscale ping" command. res is value to call cb with,
 // already partially filled.
 func (de *endpoint) cliPing(res *ipnstate.PingResult, size int, cb func(*ipnstate.PingResult)) {
@@ -433,6 +437,11 @@ func (de *endpoint) cliPing(res *ipnstate.PingResult, size int, cb func(*ipnstat
 		cb(res)
 		return
 	}
+	if size > MaxDiscoPingSize {
+		res.Err = errPingTooBig.Error()
+		cb(res)
+		return
+	}
 
 	now := mono.Now()
 	udpAddr, derpAddr, _ := de.addrForSendLocked(now)
@@ -457,6 +466,7 @@ func (de *endpoint) cliPing(res *ipnstate.PingResult, size int, cb func(*ipnstat
 var (
 	errExpired     = errors.New("peer's node key has expired")
 	errNoUDPOrDERP = errors.New("no UDP or DERP addr")
+	errPingTooBig  = errors.New("ping size too big")
 )
 
 func (de *endpoint) send(buffs [][]byte) error {
@@ -564,13 +574,9 @@ const discoPingSize = len(disco.Magic) + key.DiscoPublicRawLen + disco.NonceLen
 // The caller should use de.discoKey as the discoKey argument.
 // It is passed in so that sendDiscoPing doesn't need to lock de.mu.
 func (de *endpoint) sendDiscoPing(ep netip.AddrPort, discoKey key.DiscoPublic, txid stun.TxID, size int, logLevel discoLogLevel) {
-	padding := 0
-	if size > int(tstun.DefaultMTU()) {
-		size = int(tstun.DefaultMTU())
-	}
-	if size-discoPingSize > 0 {
-		padding = size - discoPingSize
-	}
+	size = min(size, MaxDiscoPingSize)
+	padding := max(size-discoPingSize, 0)
+
 	sent, _ := de.c.sendDiscoMessage(ep, de.publicKey, discoKey, &disco.Ping{
 		TxID:    [12]byte(txid),
 		NodeKey: de.c.publicKeyAtomic.Load(),