Split vm_page_free_toq() into two parts, preparation vm_page_free_prep()
and insertion into the phys allocator free queues vm_page_free_phys().
Also provide a wrapper vm_page_free_phys_pglist() for batched free.

Reviewed by: alc, markj
Tested by: mjg (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week