much longer keepalive timer (don't start probes for 2 hr);

much longer keepalive timer (don't start probes for 2 hr);
fix input window test again (accept acks from FINS at left edge and responses
to keepalives, but dropafterack otherwise; also fixes Phil Karn's problem
with failure to respond to new SYN to left of window); can't use tcp_drop
after RST, or we'll send RST in response

SCCS-vsn: sys/netinet/tcp_input.c 7.17
SCCS-vsn: sys/netinet/tcp_timer.c 7.12
SCCS-vsn: sys/netinet/tcp_timer.h 7.5
SCCS-vsn: sys/netinet/tcp_usrreq.c 7.8