iptables: bump to version 1.6.0

Add option for nftables compat utilies and for bpf compiler/nfsynproxy
configuration tool.

Drop symlink trickery since it's not required any more.

Switch homepage to proper one and drop the kernel v2.4+ note since
that's not supported by buildroot anyway.

Drop most patches (except musl build) since they're upstream.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>