[NETFILTER]: nf_nat: add FTP NAT helper port

Add FTP NAT helper.

Split out from Jozsef's big nf_nat patch with a few small fixes by myself.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>