trim: Target SCSI UNMAP command instead of WRITE SAME.

* The UNMAP command is theoretically much faster, as it can get many block
  ranges instead of just a single range.
* Furthermore, the ATA TRIM command resembles it much better.
* Therefore, fs_trim_data now gets an array of ranges, and we use SCSI UNMAP
  to trim.
* Updated BFS code to collect array ranges to fully support the new
  fs_trim_data possibilities.

headers/os/drivers/Drivers.h

@@ -172,9 +172,12 @@ typedef struct {
 
 /* B_TRIM_DEVICE data structure */
 typedef struct {
-	off_t	offset;					/* offset (in bytes) */
-	off_t	size;
-	off_t	trimmed_size;			/* filled on return */
+	uint32	range_count;
+	uint64	trimmed_size;			/* filled on return */
+	struct range {
+		uint64	offset;				/* offset (in bytes) */
+		uint64	size;
+	} ranges[1];
 } fs_trim_data;