ofp-util: Again allow OpenFlow port 0 in commands and actions.

ofp-util: Again allow OpenFlow port 0 in commands and actions.

Commit c6100d92142b (ovs-ofctl: Accept port keywords, OF1.1 port numbers,
reject port number 0.) intentionally prohibited using port 0, which is not
a valid OpenFlow port number, in OpenFlow commands and actions.  However,
since then we have discovered that the NVP controller uses port 0 in
"resubmit" actions and hence in flow matches as well.  This seems like a
valid enough use case; after all, the port number in a resubmit action is
really just an arbitrary number that need not (and preferably does not)
refer to an actual OpenFlow port.  Therefore, this commit restores the
ability to use port 0.

(Only functions that parse OpenFlow commands and actions from strings, not
those that interpret binary OpenFlow coming in on OpenFlow connections,
were affected by this prohibition, so this only comes up in corner cases
anyhow.)

Reported-by: Gurucharan Shetty <gshetty@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>