IKEv2 REDIRECT mechanism with redirection loop.

west wants to connect to east but is redirected to north instead. but
north is configured to redirect west to east, and west falls into a redirect
loop. (redirect is done in IKE_AUTH replies)

normally west has a hardcoded route for eastnet (unrelated to libreswan)
to make testing blocking of unencrypted packets easier. But we cannot have
this route now because we need to send those packets to north instead.