pcap-file: Allow capturing TCP streams where the SYN is not seen.

Until now, the tcp_stream() code has ignored any TCP packets that don't
correspond to a stream that began with a TCP SYN.  This commit changes the
code so that any TCP packet that contains a payload starts a new stream.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Reported-by: Vasu Dasari <vdasari@gmail.com>