Default value for MaxQueueLen

[florianl]

In [1] a default value is defined for the NfQueue maximum length. If not set explicit, the default value for this parameter [2] in this Go implementation is 0. This can result in unexpected behaviour. Therefore, find a way to check if MaxQueueLen is set - if not use the kernel default value.

[1] https://github.com/torvalds/linux/blob/cd8dead0c39457e58ec1d36db93aedca811d48f1/net/netfilter/nfnetlink_queue.c#L51
[2] https://github.com/florianl/go-nfqueue/blob/master/types.go#L22