This fixes: https://github.com/NixOS/nixpkgs/issues/119066.
When `djbdns` is repeatedly queried it will crash with:
```
mmap: tinydns (842): VmData 331776 exceed data ulimit 300000. Update limits or use boot option ignore_rlimit_data.
```
So we increase the softlimit from 300000 to 4500000 as suggested in
the issue.
The `tinydns` NixOS test has been extended with testing for the faulty
behaviour.
