Add support for connection tracking helper/ALGs.

Add support for connection tracking helper/ALGs.

This patch adds support for specifying a "helper" or ALG to assist
connection tracking for protocols that consist of multiple streams.
Initially, only support for FTP is included.

Below is an example set of flows to allow FTP control connections from
port 1->2 to establish active data connections in the reverse direction:

    table=0,priority=1,action=drop
    table=0,arp,action=normal
    table=0,in_port=1,tcp,action=ct(alg=ftp,commit),2
    table=0,in_port=2,tcp,ct_state=-trk,action=ct(table=1)
    table=1,in_port=2,tcp,ct_state=+trk+est,action=1
    table=1,in_port=2,tcp,ct_state=+trk+rel,action=ct(commit),1

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>