firewall: allow local redirection of ports

Allow a redirect like:

config redirect
        option src 'wan'
        option dest 'lan'
        option src_dport '22001'
        option dest_port '22'
        option proto 'tcp'

note the absence of the "dest_ip" field, meaning to terminate the connection on the firewall itself.

This patch makes three changes:

(1) moves the conntrack module into the conntrack package (but not any of the conntrack_* helpers).
(2) fixes a bug where the wrong table is used when the "dest_ip" field is absent.
(3) accepts incoming connections on the destination port on the input_ZONE table, but only for DNATted
    connections.

In the above example,

ssh -p 22 root@myrouter

would fail from the outside, but:

ssh -p 22001 root@myrouter

would succeed.  This is handy if:

(1) you want to avoid ssh probes on your router, or
(2) you want to redirect incoming connections on port 22 to some machine inside your firewall, but
    still want to allow firewall access from outside.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

SVN-Revision: 26617