nft: Implement nft_chain_foreach()

nft: Implement nft_chain_foreach()

This is just a fancy wrapper around nftnl_chain_list_foreach() with the
added benefit of detecting invalid table names or uninitialized chain
lists. This in turn allows to drop the checks in flush_rule_cache() and
ignore the return code of nft_chain_foreach() as it fails only if the
dropped checks had failed, too.

Since this wrapper does the chain list lookup by itself, use of
nft_chain_list_get() shrinks down to a single place, namely inside
nft_chain_find(). Therefore fold it into the latter.

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