Fix some required -j target rules delete bug when delete source/destinat...

These rules must be exists in raw table when source nat or destination nat is enabled.

 -A PREROUTING -j VYATTA_CT_HELPER
 -A PREROUTING -j NAT_CONNTRACK
 -A OUTPUT -j VYATTA_CT_HELPER
 -A OUTPUT -j NAT_CONNTRACK
 -A NAT_CONNTRACK -j ACCEPT

But, when you delete one of pair completely in the state of exists both, these rules will be deleted despite the required.

See below...

 [edit]
 vyos@vyos# show nat
 Configuration under specified path is empty
 [edit]
 vyos@vyos# set nat source rule 1 outbound-interface eth0
 [edit]
 vyos@vyos# set nat source rule 1 translation address masquerade 
 [edit]
 vyos@vyos# commit
 [edit]
 vyos@vyos# sudo iptables-save > /tmp/1         
 [edit]
 vyos@vyos# set nat destination rule 1 inbound-interface eth0
 [edit]
 vyos@vyos# set nat destination rule 1 translation address 172.16.1.1
 [edit]
 vyos@vyos# commit
 [edit]
 vyos@vyos# delete nat destination 
 [edit]
 vyos@vyos# commit
 [edit]
 vyos@vyos# sudo iptables-save > /tmp/2
 [edit]
 vyos@vyos# diff -u /tmp/1 /tmp/2
 --- /tmp/1	2014-05-30 02:04:24.657329323 +0000
 +++ /tmp/2	2014-05-30 02:05:11.837330363 +0000
 ... snip ...
 @@ -37,30 +37,24 @@
  -A VYATTA_PRE_FW_IN_HOOK -j RETURN
  -A VYATTA_PRE_FW_OUT_HOOK -j RETURN
  COMMIT
 -# Completed on Fri May 30 02:04:24 2014
 -# Generated by iptables-save v1.4.12.2 on Fri May 30 02:04:24 2014
 +# Completed on Fri May 30 02:05:11 2014
 +# Generated by iptables-save v1.4.12.2 on Fri May 30 02:05:11 2014
  *raw
 -:PREROUTING ACCEPT [0:0]
 -:OUTPUT ACCEPT [0:0]
 -:NAT_CONNTRACK - [0:0]
 +:PREROUTING ACCEPT [28:2096]
 +:OUTPUT ACCEPT [16:1492]
  :VYATTA_CT_HELPER - [0:0]
  :VYATTA_CT_IGNORE - [0:0]
  :VYATTA_CT_OUTPUT_HOOK - [0:0]
  :VYATTA_CT_PREROUTING_HOOK - [0:0]
  :VYATTA_CT_TIMEOUT - [0:0]
  -A PREROUTING -j VYATTA_CT_IGNORE
 --A PREROUTING -j VYATTA_CT_HELPER
  -A PREROUTING -j VYATTA_CT_TIMEOUT
  -A PREROUTING -j VYATTA_CT_PREROUTING_HOOK
 --A PREROUTING -j NAT_CONNTRACK
  -A PREROUTING -j NOTRACK
  -A OUTPUT -j VYATTA_CT_IGNORE
 --A OUTPUT -j VYATTA_CT_HELPER
  -A OUTPUT -j VYATTA_CT_TIMEOUT
  -A OUTPUT -j VYATTA_CT_OUTPUT_HOOK
 --A OUTPUT -j NAT_CONNTRACK
  -A OUTPUT -j NOTRACK
 --A NAT_CONNTRACK -j ACCEPT
  -A VYATTA_CT_HELPER -p tcp -m tcp --dport 1525 -j CT --helper tns
  -A VYATTA_CT_HELPER -p tcp -m tcp --dport 1521 -j CT --helper tns
  -A VYATTA_CT_HELPER -p udp -m udp --dport 111 -j CT --helper rpc
 @@ -71,4 +65,4 @@
  -A VYATTA_CT_PREROUTING_HOOK -j RETURN
  -A VYATTA_CT_TIMEOUT -j RETURN
  COMMIT
 -# Completed on Fri May 30 02:04:24 2014
 +# Completed on Fri May 30 02:05:11 2014