Introduce options IP[6]TABLES_RESTORE_NOFLUSH
Debian bug 998416:
iptables-persistent: option for not flushing existing rules when starting:
- d/netfilter-persistent.default: introduce IP[6]TABLES_RESTORE_NOFLUSH
- p/15-ip4tables: check IPTABLES_RESTORE_NOFLUSH
- p/25-ip6tables: check IP6TABLES_RESTORE_NOFLUSH; source config file
(check the right variable: IP*6*TABLES_SKIP_SAVE while still there)
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>