iptables: Add file output option to iptables-save

iptables: Add file output option to iptables-save

Adds an option to output the results of iptables-save,
ip6tables-save, and xtables-save save to a file.
Updates the man page with this new option.

Uses the dup2 call to replace stdout with the specified file.
Error output is unchanged.

This is a feature requested by a Gentoo developer in
Bugzilla #905.

Signed-off-by: Oliver Ford <ojford@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>