Changes

# * ''shmem mapping'' — a mapping of file belonging to <code>tmpfs</code>. UBC accounts these pages separately and the description below doesn't take such pages into account;# * ''private mapping'' — this includes the following types of mappings:## ** writable anonymous mappings## ** writable private file mappings#: Both types are not backed by disk file and thus may not be just freed. These mappings are charged with possible reject right when they are made — in <code>sys_mmap()</code>/<code>sys_brk()</code>.# * ''unused pages'' — this is the number of pages which belong to private mapping, but are not yet touched.