tcp: Allow per-tcpcb keepintvl and keepcnt
- Expose tcp_keepcnt
- Add t_keepintvl, t_keepcnt and t_maxidle to tcpcb. t_keepintvl and
t_keepcnt is initialized to tcp_keepintvl and tcp_keepcnt respectively.
t_maxidle is initialized to t_keepcnt * t_keepintvl
- The accepted socket's t_keepintvl, t_keepcnt and t_maxidle is inherited
from the listen socket
- Add IPPROTO_TCP/TCP_KEEPINTVL and IPPROTO_TCP.TCP_KEEPCNT socket options
to get and set t_keepintvl and t_keepcnt. TCP_KEEPINTVL's unit is
milliseconds, which is as same as the unit of the sysctl node
net.inet.tcp.keepintvl
- Remove no longer used tcp_maxidle