l2tp update and fix mtu

[Robert Langhammer]

[PATCH 1/2] Update tunneldigger

There are a few more patches left.

[PATCH 2/2] fff-tunneldigger: set fix MTU for l2tp tunnel.

Active pmtu is definitely preferable, but we have seen CPU load issues when many tunnels are connected to a broker. This can be fixed with a fixed mtu.
When pmtu is disabled at the broker, the tunneldigger sets the mtu to 1488, regardless of the broker's settings.
This mtu is too big. This results in a considerable (up to 80% I've measured) break in the upload throughput.
A little too small mtu is irrelevant. Since only very few tunnels have a mtu <1438, this value seems to me to be a good compromise.
If pmtu is enabled, this patch has no effect.

Robert