stub was confusing mk by leaving fds open strangely

Reference: /n/atom/patch/applied/stubclosefd
Date: Tue Apr 15 23:57:20 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/aux/stub.c	Tue Apr 15 23:57:20 2014
+++ /sys/src/cmd/aux/stub.c	Tue Apr 15 23:57:20 2014
@@ -189,6 +189,10 @@
 		*p++ = '\0';
 		kidname = p;
 	}
+	/* don't leave standard descriptors open to confuse mk */
+	close(0);
+	close(1);
+	close(2);
 	postmountsrv(&fs, nil, mtpt, MBEFORE);
 	exits(nil);
 }