8 is better than 10

It's quite common for the real pmtu to be 8 less than what was
advertised.

docs/architecture.txt

@@ -204,6 +204,6 @@ and gets an error informing it of a new PMTU, it will attempt to
 verify that PMTU by sending packets of exactly that size to the remote
 peer. If it gets no indication from the remote weave that these
 packets have been received within a timeout period, it will
-artificially drop the PMTU by 10 and try again. Thus this will slowly
+artificially drop the PMTU by 8 and try again. Thus this will slowly
 reduce the PMTU until a working PMTU is found. This is slow, but it
 does work and works around ICMP packets being dropped.

forwarder.go

@@ -146,8 +146,8 @@ func (conn *LocalConnection) forwarderLoop(forwardCh <-chan *ForwardedFrame, sto
 			// to other data.
 			pmtuVerifyTick = nil
 			if !conn.isClientPMTUVerfied() {
-				clientPMTU -= 10
-				maxPayload -= 10
+				clientPMTU -= 8
+				maxPayload -= 8
 				updateClientPMTU()
 			}
 		case frame = <-forwardCh: