Add block device tracing facility

The implementation is in libblockdriver, and works transparently for
all block drivers. The new btrace(8) tool can be used to control block
tracing; see ``man btrace'' for details.

lib/libblockdriver/driver.h

@@ -1,10 +1,13 @@
 #ifndef _BLOCKDRIVER_DRIVER_H
 #define _BLOCKDRIVER_DRIVER_H
 
+#define SINGLE_THREAD	(0)				/* single-thread ID */
+#define MAIN_THREAD	(BLOCKDRIVER_MT_MAX_WORKERS)	/* main thread ID */
+
 _PROTOTYPE( void blockdriver_handle_notify, (struct blockdriver *bdp,
 	message *m_ptr) );
 _PROTOTYPE( int blockdriver_handle_request, (struct blockdriver *bdp,
-	message *m_ptr) );
+	message *m_ptr, thread_id_t thread) );
 _PROTOTYPE( void blockdriver_reply, (message *m_ptr, int ipc_status,
 	int reply) );