linux(4): Improve netlink diagnostics

Add some missing netlink_family definitions and produce vaguely
human-readable error messages for those definitions, like we used to do for
just ROUTE and KOBJECT_UEVENTS.

Additionally, if we know it's a netfilter socket but didn't find it in the
table, fall back to printing that instead of the generic handler ("socket
domain 16, ...").

No change to the emulator correctness, just mildly improved diagnostics for
gaps.