VM: make mapping types explicit

VM: make mapping types explicit

Introduce explicit abstractions for different mapping types,
handling the instantiation, forking, pagefaults and freeing of
anonymous memory, direct physical mappings, shared memory and
physically contiguous anonymous memory as separate types, making
region.c more generic.

Also some other genericification like merging the 3 munmap cases
into one.

COW and SMAP safemap code is still implicit in region.c.