iptables-restore/save: exit when given an unknown option

iptables-restore/save: exit when given an unknown option

When an unknown option is given, iptables-restore should exit instead of
continue its operation. For example, if `--table` was misspelled, this
could lead to an unwanted change. Moreover, exit with a status code of
1. Make the same change for iptables-save.

OTOH, exit with a status code of 0 when requesting help.

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>