If passt forwards many or all ports, it needs to open a relatively significant
amount of file descriptors. We're already setting the RLIMIT_NOFILE soft quota
to match the hard quota via setrlimit() in main(), but that's not necessarily
enough on some distributions.

If we can't open enough file descriptors, we'll just go ahead, bind as many
ports as we can, and warn about the rest. In pasta mode, we might fail from time
to time to establish spliced connections.

This is not necessarily desirable, because we're doing something wildly
different compared to what we were configured for. Fail gracefully instead: if
we can't bind for that reason (and not getting, say, EACCES), explain and quit,
so that the user realises right away.