Fix so that when a slice or a partition is removed through g_slice_config(),

it is destroyed in GEOM, in addition to being removed from /dev.
Before this patch, if you applied a new MBR which deleted a slice,
the deleted slice would not be in /dev, but it would still appear
in kern.geom.conftxt and kern.geom.confxml, which would confused
the diskPartitionEditor in sysinstall.

Submitted by:   pjd
Tested by:      pjd, rodrigc
MFC after:	1 week

sys/geom/geom_slice.c

@@ -339,6 +339,7 @@ g_slice_config(struct g_geom *gp, u_int idx, int how, off_t offset, off_t length
 			return (0);
 		if (bootverbose)
 			printf("GEOM: Deconfigure %s\n", pp->name);
+		pp->flags |= G_PF_WITHER;
 		g_orphan_provider(pp, ENXIO);
 		gsl->provider = NULL;
 		gsp->nprovider--;