choose an intro point, connect to it,
choose a rend point, connect to it and establish a cookie,
get an ack from the rendezvous point,
and know when both circs are ready for her.

APConns don't use conn->purpose anymore

don't initiate a renddesc lookup if one is already in progress

also fix a buffer overflow in nickname parsing (only exploitable
by the operator though)


svn:r1471