Enable the use of VM_PHYSSEG_SPARSE on amd64 and i386, making it the default
on i386 PAE.  Previously, VM_PHYSSEG_SPARSE could not be used on amd64 and
i386 because vm_page_startup() would not create vm_page structures for the
kernel page table pages allocated during pmap_bootstrap() but those vm_page
structures are needed when the kernel attempts to promote the corresponding
kernel virtual addresses to superpage mappings.  To address this problem, a
new public function, vm_phys_add_seg(), is introduced and vm_phys_init() is
updated to reflect the creation of vm_phys_seg structures by calls to
vm_phys_add_seg().

Discussed with:	Svatopluk Kraus
MFC after:	3 weeks
Sponsored by:	EMC / Isilon Storage Division