server/{keyexch,peer}.c: Maybe key-exchange messages come out of the blue.

server/{keyexch,peer}.c: Maybe key-exchange messages come out of the blue.

Don't insist in `p_read' that key-exchange messages always come from a
known address.  Instead, leave this to `kx_message' to sort out.

This involves a change of interface to `kx_message', passing in the
sender address, and passing out an indication as to whether the address
was recognized.  It also means that there's a little extra fancy
footwork involved because `kx_message' might not have a key-exchange
control block conveniently to hand.