iptables: use autoconf to process .in man pages

iptables: use autoconf to process .in man pages

This fixes a bug in iptables.8 and ip6tables.8 where @PACKAGE_VERSION@
was not processed in the VERSION section. It also simplifies the
Makefile by avoiding some sed commands.

[ Mangled this patch to rename iptables-extensions.8.in to
  iptables-extensions.8.tmpl.in to avoid having a file whose name
  is terminated by .in.in --pablo ]

Signed-off-by: Andy Spencer <andy753421@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>