Add a cn_unit and cn_name field to the consdev structure.

Most of the actual use of the cn_dev field is merely to get the name,
and most of the actual initializations are bogusly using makedev()
because the probe/attach has not been completed.

Instead we will migrate console drivers to fill in the name and if
the driver needs it: the unit number, thereby avoiding the bogus
calls to makedev().

sys/sys/cons.h

@@ -70,6 +70,8 @@ struct consdev {
 	dev_t	cn_dev;		/* major/minor of device */
 	short	cn_pri;		/* pecking order; the higher the better */
 	void	*cn_arg;	/* drivers method argument */
+	int	cn_unit;	/* some drivers prefer this */
+	char	cn_name[SPECNAMELEN + 1];	/* console (device) name */
 };
 
 /* values for cn_pri - reflect our policy for console selection */