Make out-of-swap checks more robust.

Make out-of-swap checks more robust.

Consider that the swap space is full when 99% of it is filled with pages that
are no longer present in memory.  This prevents deadlocks when out-of-swap if
some swap ranges had I/O errors and have been marked as 'bad', or if some
pages are unreachable by the pagedaemon and still holding some slots.

Also introduce uvm_swapisfilled() to check if there are some free slots in
the swap.  Note that we consider the swap space completly filled if it is not
possible to write a full cluster.  This prevents deadlocks if a few slots are
never allocated.

ok miod@