src: cache gets out of sync in interactive mode

src: cache gets out of sync in interactive mode

Since 94a945ffa81b ("libnftables: Get rid of explicit cache flushes"),
the cache logic checks for the generation number to refresh the cache.

This breaks interactive mode when listing stateful objects though. This
patch adds a new flag to force a cache refresh when the user requests a
ruleset listing.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>