xauth with first east->road XCHG-MODE-CFG request (4th incoming message) dropped

In main mode: there are 3 ISAKMP_XCHG_IDPROT exchanges and then east
sends XCHG_MODE_CFG request to road - the 4th incoming message.

East should retransmit both the XCHG_MODE_CFG request and the previous
exchange response.
