nft: Delete builtin chains compatibly

nft: Delete builtin chains compatibly

Attempting to delete all chains if --delete-chain is called without
argument has unwanted side-effects especially legacy iptables users are
not aware of and won't expect:

* Non-default policies are ignored, a previously dropping firewall may
  start accepting traffic.

* The kernel refuses to remove non-empty chains, causing program abort
  even if no user-defined chain exists.

Fix this by requiring a rule cache in that situation and make builtin
chain deletion depend on its policy and number of rules. Since this may
change concurrently, check again when having to refresh the transaction.

Also, hide builtin chains from verbose output - their creation is
implicit, so treat their removal as implicit, too.

When deleting a specific chain, do not allow to skip the job though.
Otherwise deleting a builtin chain which is still in use will succeed
although not executed.

Fixes: 61e85e3192dea ("iptables-nft: allow removal of empty builtin chains")
Signed-off-by: Phil Sutter <phil@nwl.cc>