nft: cache: Fetch cache for specific chains

nft: cache: Fetch cache for specific chains

Iterate over command list and collect chains to cache. Insert them into
a sorted list to pass to __nft_build_cache().

If a command is interested in all chains (e.g., --list), cmd->chain
remains unset. To record this case reliably, use a boolean
('all_chains'). Otherwise, it is hard to distinguish between first call
to nft_cache_level_set() and previous command with NULL cmd->chain
value.

When caching only specific chains, manually add builtin ones for the
given table as well - otherwise nft_xt_builtin_init() will act as if
they don't exist and possibly override non-default chain policies.

Signed-off-by: Phil Sutter <phil@nwl.cc>