vm: Introduce VM_ALLOC_NOFREE and PG_NOFREE

This patch adds two additional vm_page flags to distinguish pages that
never get released while the system is running (e.g. UMA_ZONE_NOFREE slabs).

Differential Revision: https://reviews.freebsd.org/D45970
Reviewed by: alc, kib, markj
Tested by: alc