Include <sys/vmmeter.h> directly instead of relying on it being

Include <sys/vmmeter.h> directly instead of relying on it being
pulled by <uvm/uvm_extern.h> and turn uvm_total() into a private
function.

The preferred way to get memory stats is through the VM_UVMEXP
sysctl(3) since VM_METER is just a wrapper on top of it.  In the
kernel, use `uvmexp' directly instead of uvm_total().

This change does not remove <sys/vmmeter.h> from <uvm/uvm_extern.h>
to give some more time to port maintainers to fix their ports.

ok guenther@ as part of a larger diff.