xtables: rename xt-multi binaries to -nft, -legacy

xtables: rename xt-multi binaries to -nft, -legacy

This adds a clear distinction between old iptables (formerly
xtables-multi, now xtables-legacy-multi) and new iptables
(formerly xtables-compat-multi, now xtables-nft-multi).

Users will get the ip/ip6tables names via symbolic links, having
a distinct name postfix for the legacy/nft variants helps to
make a clear distinction, as iptables-nft will always use
nf_tables and iptables-legacy always uses get/setsockopt wheres
"iptables" could be symlinked to either -nft or -legacy.

Signed-off-by: Florian Westphal <fw@strlen.de>