deckard: abort if sendmmsg/recvmmsg are in use

Deckard does not support these and it leads to confusing errors.
In long term we need to migrate Deckard to different network backend:
https://gitlab.labs.nic.cz/knot/deckard/issues/42
28 jobs for deckard-mmsg-abort
in 6 minutes and 36 seconds and was queued for 6 seconds