if creating a single plan 9 partition, make it active

Reference: /n/sources/patch/applied/mbr
Date: Wed Aug  5 03:07:18 CES 2009
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/disk/mbr.c	Wed Aug  5 03:07:13 2009
+++ /sys/src/cmd/disk/mbr.c	Wed Aug  5 03:07:11 2009
@@ -105,6 +105,7 @@
 static void
 wrtentry(Disk *disk, Tentry *tp, int type, u32int base, u32int lba, u32int end)
 {
+	tp->active = 0x80;
 	tp->type = type;
 	writechs(disk, &tp->starth, lba);
 	writechs(disk, &tp->endh, end-1);
@@ -129,7 +130,7 @@
 		flag9 = 1;
 		break;
 	case 'm':
-		mbrfile = ARGF();
+		mbrfile = EARGF(usage());
 		break;
 	default:
 		usage();