- avoid malloc() if the number of fds is small.
- pack the bits better so that `small' is quite large.
- don't waste time generating zero bits for null fd_set pointers or
scanning these bits.
Possibly improved select():
- free malloc()ed storage before returning. This is simpler and I
think huge select()s aren't worth optimizing since they are rare,
relative gain would be small and there would be tiny costs for all
selects().
Reviewed by: ache (first version by him too)
301b9cec88