Simplify sendto_kdc.c

Simplify sendto_kdc.c

* Get rid of the "x" member of conn_state, which used to be a union
  but hasn't been since r14742.
* Define a structure type for the "out" member of conn_state.
* Rename incoming_krb5_message to incoming_message for brevity.
* Make the "pos" member of incoming_message an offset instead of a
  pointer, simplifying several present and future computations.
* Use "in" and "out" aliases to the conn_state in and out members
  where it improves brevity.
* Rename set_conn_state_msg_length to set_transport_message and give
  it a descriptive comment.
* Call set_transport_message from start_connection only, instead of
  once in add_connection and perhaps again in start_connection.  To
  make this possible, pass the original message argument to maybe_send
  and start_connection.
* Use make_data and empty_data helpers where appropriate.