Restore loader(8)'s ability for lsdev to show partitions within a bsd slice.

I'm pretty sure this used to work at one time, perhaps long ago.  It has
been failing recently because if you call disk_open() with dev->d_partition
set to -1 when d_slice refers to a bsd slice, it assumes you want it to
open the first partition within that slice.  When you then pass that open
dev instance to ptable_open(), it tries to read the start of the 'a'
partition and decides there is no recognizable partition type there.

This restores the old functionality by resetting d_offset to the start
of the raw slice after disk_open() returns.  For good measure, d_partition
is also set back to -1, although that doesn't currently affect anything.

I would have preferred to make disk_open() avoid such rude assumptions and
if you ask for partition -1 you get the raw slice.  But the commit history
shows that someone already did that once (r239058), and had to revert it
(r239232), so I didn't even try to go down that road.

stand/common/disk.c

@@ -133,6 +133,13 @@ ptable_print(void *arg, const char *pname, const struct ptable_entry *part)
 		dev.d_partition = -1;
 		if (disk_open(&dev, part->end - part->start + 1,
 		    od->sectorsize) == 0) {
+			/*
+			 * disk_open() for partition -1 on a bsd slice assumes
+			 * you want the first bsd partition.  Reset things so
+			 * that we're looking at the start of the raw slice.
+			 */
+			dev.d_partition = -1;
+			dev.d_offset = part->start;
 			table = ptable_open(&dev, part->end - part->start + 1,
 			    od->sectorsize, ptblread);
 			if (table != NULL) {