careful with sequence number arithmetic vs. window sizes

careful with sequence number arithmetic vs. window sizes
(u_long vs. int vs. u_short); don't lose mbufs on failure part thru new
connection setup from LISTEN; use tiflags consistently; add missing ACKNOW
in LISTEN input case; don't use window if no ACK, as TAC's send garbage;
make input urgent pointer sensible so it compares right on first URG,
accounting for seq-number wraparound.

SCCS-vsn: sys/netinet/tcp_input.c 6.19