Avoid a crash on Mac OS X 10.8 (at least) when we close libdispatch's

fds out from under it before executing the command.  Switch to just
setting the close on exec flag instead.

compat/closefrom.c

@@ -83,8 +83,14 @@ closefrom_fallback(int lowfd)
     if (maxfd < 0)
 	maxfd = OPEN_MAX;
 
-    for (fd = lowfd; fd < maxfd; fd++)
+    for (fd = lowfd; fd < maxfd; fd++) {
+#ifdef __APPLE__
+	/* Avoid potential crash with libdispatch when we close its fds. */
+	(void) fcntl(fd, F_SETFD, FD_CLOEXEC);
+#else
 	(void) close((int) fd);
+#endif
+    }
 }
 
 /*