khan: wip driver implementation
khan: wip driver implementation

Implements the connection callback, which accepts and opens up the read
end of the pipe, then just waits. Stores open connections on u3_khan.
Uses simple incrementing connection numbers.

I noticed in here that vere/newt.c seems to implement a protocol much
like the one we're looking for; may try to switch to using that more
explicitly.

Needs more well-thought-out error handling policy in connection
callback. What do we do -- bail? Deliver a non-fatal error and drop the
connection?

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