Reset the dir mode to 750 after mkdtemp(3) returns.

Mkdtemp(3) creates the directory with mode 0700, and since it
already exists, we do not change the mode subsequently, which
causes jailed with uid=vcache to keel over.

bin/varnishd/mgt/mgt_main.c

@@ -750,6 +750,7 @@ main(int argc, char * const *argv)
 			AN(p);
 			VSB_destroy(&vsb);
 			AN(mkdtemp(p));
+			AZ(chmod(p, 0750));
 			n_arg = p;
 		}
 	}