This patch streamlines poll and select by adding fast paths for a
small number of descriptors passed. The majority of polls/selects
seem to be of this nature. The main saving comes from not allocating
two pages for wait queue and table, but from using stack allocation
(upto 256bytes) when only a few descriptors are needed. This makes
it as fast again as 2.0 and even a bit faster because the wait queue
page allocation is avoided too (except when the drivers overflow it)

select also skips a lot faster over big holes and avoids the separate
pass of determining the max. number of descriptors in the bitmap.

A typical linux system saves a considerable amount of unswappable memory
with this patch, because it usually has 10+ daemons hanging around in poll or
select with each two pages allocated for data and wait queue.

Some other cleanups.