Introduce dummy pagers for 'special' subsystems using UVM objects.

Introduce dummy pagers for 'special' subsystems using UVM objects.

Some pmaps (x86, hppa) and the buffer cache rely on UVM objects to allocate
and manipulate pages.  These objects should not be manipulated by uvm_fault()
and do not currently require the same locking enforcement.

Use the dummy pagers to explicitly document which UVM functions are meant to
manipulate UVM objects (uobj) that do not need the upcoming `vmobjlock' and
instead still rely on the KERNEL_LOCK().

Tested by many as part of a larger diff.

ok kettenis@, beck@