add LogGroup and LogPrefix arguments to Log target

I suspect that a Log target without LogGroup becomes a Netfilter LOG target (ie sent to dmesg), whereas any LogGroup value makes it a Netfilter NFLOG - sent to ulogd. I can't find any trace of that feature in the documentation, but as a matter of fact, I was unable to receive logs without a LogGroup in ulogd.

Besides, this implementation has 8 possible LogGroups, but browsing the feature I saw values as loggroup 31 when called from nftables. I didn't find an upper limit. I also don't know how big the LogPrefix can be. I think I wasted quite some time scrolling netfilter documentation, whereas I guess those values are either fixed in libnftnl or in libmnl. Got a guess ? Otherwise I can keep scrolling.

Should we enforce maximum sizes or would the underlying methods do that ?