Prefer more portable test(1) constructs

* util/grub.d/00_header.in (make_timeout): Use && rather than test
-a.
* util/grub.d/10_windows.in: Likewise.
* util/grub.d/10_netbsd.in (netbsd_load_fs_module): Use || rather
than test -o.
* util/grub.d/30_os-prober.in: Use && rather than test -a, and ||
rather than test -o.

util/grub.d/00_header.in

@@ -285,7 +285,7 @@ make_timeout ()
     if [ "x${3}" != "x" ] ; then
 	timeout="${2}"
 	style="${3}"
-    elif [ "x${1}" != "x" -a "x${1}" != "x0" ] ; then
+    elif [ "x${1}" != "x" ] && [ "x${1}" != "x0" ] ; then
 	# Handle the deprecated GRUB_HIDDEN_TIMEOUT scheme.
 	timeout="${1}"
 	if [ "x${2}" != "x0" ] ; then