xtables-compat-restore: flush user-defined chains with -n

xtables-compat-restore: flush user-defined chains with -n

-n still flushes user-defined chains and its content, the following snippet:

 iptables-compat -N FOO
 iptables-compat -I INPUT
 iptables-compat -I FOO
 iptables-compat -I FOO
 iptables-compat-save > A
 iptables-compat-restore < A
 iptables-compat -N BAR
 iptables-compat -A BAR
 iptables-compat-restore -n < A

results in:

 iptables-compat-save
 # Generated by xtables-save v1.6.2 on Mon May  7 17:18:44 2018
 *filter
 :INPUT ACCEPT [0:0]
 :FORWARD ACCEPT [0:0]
 :OUTPUT ACCEPT [0:0]
 :BAR - [0:0]
 :FOO - [0:0]
 -A INPUT
 -A INPUT
 -A BAR
 -A FOO
 -A FOO
 COMMIT
 # Completed on Mon May  7 17:18:44 2018

Still, user-defined chains that are not re-defined, such as BAR, are
left in place.

Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>