remote peer shutdown when using pkcipe

We use pkcipe for several temporary VPNs from remote sites that are temporary in nature,
and I noticed that even with the addition of pkcipe, the basic behaviour of cipe shutdown
has not changed, I think perhaps it should.

In basic cipe operation, there are cogent reasons for the remote cipe end to stay up,
even when one end goes down - the persistent key/port/options of conventional cipe
make it possible/likely that another incoming UDP sesstion may connect.

However, with the more session oriented pkcipe, there will by definition not be any
other session connecting with the same key/port parameters. Keeping the interface up
(which is done just prior to replacing it with the interface for the new session) has
no value, and perhaps is an opportunistic potential security opening (albeit diminishingly
small).

There is a CT_KILL mechanism in the protocol - perhaps when started up with pkcipe,
a command line flag could be used to set a mode (and perhaps there is value in making
it a general option for other circumstances too) where by which instead of just printing
out a debug message when the remote peer shuts down - it sends a CT_KILL to cause a shutdown
of the remote end.

What do people think?

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