io, seg: we should exits(nil) at the end of main

sys/src/cmd/io.c

@@ -52,4 +52,5 @@ main(int argc, char** argv) {
 			((vlong)datab[7] << 56);
 		print("0x%llx\n", data);
 	}
-}
+	exits(nil);
+}

sys/src/cmd/seg.c

@@ -44,4 +44,5 @@ main(int argc, char** argv) {
 		if(size >= 4) data |= (seg[port+2] << 16) | (seg[port+3] << 24);
 		print("0x%ulx\n", data);
 	}
-}
+	exits(nil);
+}