ikev2: Return to the original host if connection fails after redirection

ikev2: Return to the original host if connection fails after redirection

If we fail connecting to the host we got redirected to, we should restart
with the original host where we might get redirected to a different host.

We must not reset this when retrying due to INVALID_KE_PAYLOAD or COOKIE
notifies.  Since we keep the initiator SPI in those cases, we use that
flag as indicator.

Since we don't store the original remote_host value, we can't restore
that.  So there is a potential conflict with MIPv6.

Closes strongswan/strongswan#171.