Fix: Infinite fallback - replace csum_error by send_mp_fail

Otherwise we will not stop sending mp_fails, because csum_error is set
to 1.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Allow to resend on same subflow if all possibilities expired

We must allow resending on the same subflow, if no other possibility
exists. Otherwise, we cannot reinject data that is missing at the
destination through a middlbox that randomly removes DSS-options.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>