iptables: improve chain name validation

iptables: improve chain name validation

As pointed out by Andrew Domaszek, iptables allows whitespace to be included in
chain names.  This causes issues with iptables-restore, and later iptables
actions on the chain.  Attached patch disallows whitespace, and also consolidates
all chain name checking into a new function.

This closes netfilter bugzilla #855.

[ Included ip6tables changed as well --pablo ]

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>