netlink_delinerize: add more restrictions on meta nfproto removal

netlink_delinerize: add more restrictions on meta nfproto removal

We can't remove 'meta nfproto' dependencies for all cases.
Its removed for ip/ip6 families, this works fine.

But for others, e.g. inet, removal is not as simple.
For example

   meta nfproto ipv4 ct protocol tcp

is listed as 'ct protocol tcp', even when this is uses in the inet
table.

Meta L4PROTO removal checks were correct, but refactor this
into a helper function to split meta/ct checks from the common
calling function.

Ct check was lacking, we need to examine ct keys more closely
to figure out if they need to retain the network protocol depenency
or not.  Elide for NFT_CT_SRC/DST and its variants, as those imply
the network protocol to use, all others must keep it as-is.

Also extend test coverage for this.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1783
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>