Use a simpler "formula" to decide a default kern.maxthread value;

Use a simpler "formula" to decide a default kern.maxthread value;
just 2*NPROCESS rather than NPROCESS+8*MAXUSERS. Results in a slightly
higher maxthread value - the previous value was fairly likely to be
exceeded on a system running a couple of heavily threaded processes.

>                             previous        new
> MAXUSERS    NPROCESS        maxthread       (2*NPROCESS)
> 80          1310            1950            2620
> 64          1054            1566            2108
> 32          542             798             1084

ok kettenis@