add shared mapped file (in progress)

implemented:
- Pages can be shared between maps.
- A change made to a map is written to the file, at munmap().

not yet implemented:
- VM operation during page IO execution.
  Because page IO is executed with VM's lock.
- Page IO, which does not change a file size with any case.
  When munmap() races with truncate(), the file size may be changed
  illegally.

kernel/include/pager.h

@@ -7,6 +7,7 @@ enum pager_op {
 	PAGER_REQ_CREATE =	0x0001,
 	PAGER_REQ_RELEASE =	0x0002,
 	PAGER_REQ_READ =	0x0003,
+	PAGER_REQ_WRITE =	0x0004,
 };
 
 /*