VFS - no CANCEL for async non-blocking operations

- if an operation (R, W, IOCTL) is non blocking, a flag is set
  and sent to the device.

- nothing changes for sync devices

- asyn devices should reply asap if an operation is non-blocking.
  We must trust the devices, but we had to trust them anyway to
  reply to CANCEL correctly

- we safe sending CANCEL commands to asyn devices. This greatly
  simplifies the protocol. Asynchronous devices can always reply
  when a reply is ready and do not need to deal with other
  situations

- currently, none of our drivers use the flags since they drive
  virtual devices which do not block

include/minix/chardriver.h

@@ -11,7 +11,7 @@ struct chardriver {
   _PROTOTYPE( struct device *(*cdr_prepare), (dev_t device) );
   _PROTOTYPE( int (*cdr_transfer), (endpoint_t endpt, int opcode,
 	u64_t position, iovec_t *iov, unsigned int nr_req,
-	endpoint_t user_endpt) );
+	endpoint_t user_endpt, unsigned int flags) );
   _PROTOTYPE( void (*cdr_cleanup), (void) );
   _PROTOTYPE( void (*cdr_alarm), (message *m_ptr) );
   _PROTOTYPE( int (*cdr_cancel), (message *m_ptr) );

include/minix/com.h

@@ -225,6 +225,9 @@
 #define HIGHPOS		m2_l2	/* file offset (high 4 bytes) */
 #define ADDRESS 	m2_p1	/* core buffer address */
 #define IO_GRANT 	m2_p1	/* grant id (for DEV_*_S variants) */
+#define FLAGS		m2_s1   /* operation flags */
+
+#define FLG_OP_NONBLOCK	0x1 /* operation is non blocking */
 
 /* Field names for DEV_SELECT messages to character device drivers. */
 #define DEV_MINOR	m2_i1	/* minor device */