vm_page: Add a new page allocator interface for unnamed pages

The diff adds vm_page_alloc_noobj() and vm_page_alloc_noobj_domain().
These mostly correspond to vm_page_alloc() and vm_page_alloc_domain()
when no VM object is specified, with the exception that they handle
VM_ALLOC_ZERO by zeroing the page, rather than by preserving PG_ZERO.

This simplifies callers and will permit simplification of the
vm_page_alloc_domain() definition.

Since the new allocator variant is similar to vm_page_alloc_freelist(),
implement both of them using a common backend allocator function. No
functional change intended.

Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b498f71bc56af0069d9a4685b8385ee613a00727)