Fix netstat output of uses of current SYN cache left.

Fix netstat output of uses of current SYN cache left.

TCP syn cache variable scs_use is basically counting packet insertions
into syn cache.  Prefer type long to exclude overflow on fast
machines.  Due to counting downwards from a limit, it can become
negative.  Copy it out as tcps_sc_uses_left via sysctl, and print
it as signed long long integer.

OK mvs@