exit() seems to work, but needs more testing.

- Added cleaner allocation of shm addresses by moving the allocation to do_mmap().
- Added deletion routine for all objects: shadow, vm_file of type vfs_file, shm_file, etc.
  - Need to make sure objects get deleted properly after exit().
- Currently we allow a single, unique virtual address for each shm segment.

tasks/mm0/include/vm_area.h

@@ -143,6 +143,7 @@ struct vm_file {
 	unsigned long length;
 	unsigned int type;
 	struct vm_object vm_obj;
+	void (*destroy_priv_data)(struct vm_file *f);
 	void *priv_data;	/* Device pagers use to access device info */
 };