Set the active flag in the PMBR when we install bootcode on a GPT

partitioned disk.  Some BIOS require this to be set before they will
boot the device.

Approved by:	marcel
MFC after:	2 weeks

sys/geom/part/g_part_gpt.c

@@ -382,6 +382,9 @@ g_part_gpt_bootcode(struct g_part_table *basetable, struct g_part_parms *gpp)
 	codesz = MIN(codesz, gpp->gpp_codesize);
 	if (codesz > 0)
 		bcopy(gpp->gpp_codeptr, table->mbr, codesz);
+
+	/* Mark the PMBR active since some BIOS require it */
+	table->mbr[DOSPARTOFF] = 0x80;		/* status */
 	return (0);
 }