Make several amendments to the paging code to facilitate process memory management:

Make several amendments to the paging code to facilitate process memory management:
 - Move definition of struct region to kernel-wide arch.h header
 - Make page directories refer to their regions through a pointer array rather than a linked list
 - Add pg_dir_foreach_region() function for iterating over the mappings in a page directory
 - Add pg_dir_map_region() function for mapping a region from one page directory into another
 - Add a reference counter to each region
 - Add flags to regions so they can be marked as private, shared, or kernel-related