[style] parse_iptables: move to iptables.c

This lets us hide some internal details of iptables.c,
like struct iptables_s.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
[style] iptables.c: some refactoring

1. Rename iptables to _g_iptables

2. Rename find_ipt() to find_ipt_by_name()

3. Add struct iptables_s* parameter to find_ipt_by_name().

4. Switch from ARRAY_SIZE loops to "!= NULL" like it was
   before commit 8036cd3.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>