src: restore interface to index cache

src: restore interface to index cache

nftables used to have a cache to speed up interface name <-> index lookup,
restore it using libmnl.

This reduces netlink traffic since if_nametoindex() and if_indextoname() open,
send a request, receive the list of interface and close a netlink socket for
each call.  I think this is also good for consistency since nft -f will operate
with the same index number when reloading the ruleset.

The cache is populated by when nft_if_nametoindex() and nft_if_indextoname()
are used for first time. Then, it it released in the output path. In the
interactive mode, it is invalidated after each command.

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