netfilter: nf_conntrack_amanda: add support for STATE streams

The Amanda CONNECT command has been updated to establish an optional
fourth connection [0]. Previously, a CONNECT command would look like:

    CONNECT DATA port0 MESG port1 INDEX port2

nf_conntrack_amanda analyses the CONNECT command string in order to
learn the port numbers of the related DATA, MESG and INDEX streams. As
of amanda v3.4, the CONNECT command can advertise an additional port:

    CONNECT DATA port0 MESG port1 INDEX port2 STATE port3

The new STATE stream is not handled, thus the connection on the STATE
port cannot be established.

The patch adds support for STATE streams to the amanda conntrack helper.

I tested with max_expected = 3, leaving the other patch hunks
unmodified. Amanda reports "connection refused" and aborts. After I set
max_expected to 4, the backup completes successfully.

[0] https://github.com/zmanda/amanda/commit/3b8384fc9f2941e2427f44c3aee29f561ed67894#diff-711e502fc81a65182c0954765b42919eR456



Signed-off-by: Florian Tham <tham@fidion.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>