Added copy-on-write shared memory but untested yet.

For anonymous shm, mmap now adds a shm_file and devzero behind it
as two vm_objects. Faults are handled by copy_on_write(). Just as
shadows copy r/w pages from original files, it should copy r/w
pages from devzero into the shm_file in front.

shmat/shmget uses mmap to set-up their areas.

Untested yet so bugs expected.

	modified:   tasks/libl4/src/init.c
	modified:   tasks/mm0/include/shm.h
	modified:   tasks/mm0/include/vm_area.h
	modified:   tasks/mm0/src/fault.c
	modified:   tasks/mm0/src/mmap.c
	modified:   tasks/mm0/src/shm.c

tasks/mm0/include/vm_area.h

@@ -42,6 +42,7 @@ enum VM_FILE_TYPE {
 	VM_FILE_DEVZERO = 1,
 	VM_FILE_REGULAR,
 	VM_FILE_BOOTFILE,
+	VM_FILE_SHM,
 };
 
 /* Defines the type of object. A file? Just a standalone object? */