xtables: display legacy/nf_tables flavor in error messages, too

xtables: display legacy/nf_tables flavor in error messages, too

Also, in nf_tables backend case, only show more than one error
if we're iptables-restore, else we get very long concatenated errorline.

old:
iptables v1.6.2: can't initialize iptables table `security': Table does not exist (do you need to insmod?)
iptables v1.6.2: iptables: CHAIN_ADD failed (Device or resource busy): chain PREROUTINGCHAIN_ADD failed (Device or resource busy): chain INPUTCHAIN_ADD failed (Device or resource busy): chain POSTROUTINGCHAIN_ADD failed (Device or resource busy): chain OUTPUT
iptables-restore v1.6.2: iptables-restore:
line 1: CHAIN_ADD failed (Device or resource busy): chain PREROUTING
line 1: CHAIN_ADD failed (Device or resource busy): chain INPUT
line 1: CHAIN_ADD failed (Device or resource busy): chain POSTROUTING
line 1: CHAIN_ADD failed (Device or resource busy): chain OUTPUT
line 6: RULE_INSERT failed (No such file or directory): rule in chain PREROUTING

now:

iptables v1.6.2 (legacy): can't initialize iptables table `security': Table does not exist (do you need to insmod?)
iptables v1.6.2 (nf_tables):  CHAIN_ADD failed (Device or resource busy): chain PREROUTING
iptables-restore v1.6.2 (nf_tables):
line 1: CHAIN_ADD failed (Device or resource busy): chain PREROUTING
line 1: CHAIN_ADD failed (Device or resource busy): chain INPUT
line 1: CHAIN_ADD failed (Device or resource busy): chain POSTROUTING
line 1: CHAIN_ADD failed (Device or resource busy): chain OUTPUT
line 6: RULE_INSERT failed (No such file or directory): rule in chain PREROUTING

Signed-off-by: Florian Westphal <fw@strlen.de>