tcp_flags=flags/mask
        Bitwise  match on TCP flags.  The flags and mask are 16-bit num‐
        bers written in decimal or in hexadecimal prefixed by 0x.   Each
        1-bit  in  mask requires that the corresponding bit in port must
        match.  Each 0-bit in mask causes the corresponding  bit  to  be
        ignored.

        TCP  protocol  currently  defines  9 flag bits, and additional 3
        bits are reserved (must be transmitted as zero), see  RFCs  793,
        3168, and 3540.  The flag bits are, numbering from the least
        significant bit:

        0: FIN No more data from sender.

        1: SYN Synchronize sequence numbers.

        2: RST Reset the connection.

        3: PSH Push function.

        4: ACK Acknowledgement field significant.

        5: URG Urgent pointer field significant.

        6: ECE ECN Echo.

        7: CWR Congestion Windows Reduced.

        8: NS  Nonce Sum.

        9-11:  Reserved.

        12-15: Not matchable, must be zero.
Signed-off-by: NJarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: NJesse Gross <jesse@nicira.com>