Make the maximum number of vnodes a function of both the physical memory

Make the maximum number of vnodes a function of both the physical memory
size and the kernel's heap size, specifically, vm_kmem_size.  This
function allows a maximum of 40% of the vm_kmem_size to be used for
vnodes and vm objects.  This is a conservative bound based upon recent
problem reports.  (In other words, a slight increase in this percentage
may be safe.)

Finally, machines with less than ~3GB of RAM should be unaffected
by this change, i.e., the maximum number of vnodes should remain
the same.  If necessary, machines with 3GB or more of RAM can increase
the maximum number of vnodes by increasing vm_kmem_size.

Desired by:	scottl
Tested by:	jake
Approved by:	re (rwatson,scottl)