Hurd: fix the _CS_PATH getconf value

Create a new Hurd-specific confstr.h including sysdeps/unix/confstr.h
directly to skip sysdeps/unix/bsd/confstr.h (which adds a non-existing
/usr/ucb).

2012-10-03  Pino Toscano  <toscano.pino@tiscali.it>

	* sysdeps/mach/hurd/confstr.h: New file.
--- /dev/null
+++ b/sysdeps/mach/hurd/confstr.h
@@ -0,0 +1,2 @@
+/* We do not want the sysdeps/unix/bsd version. */
+#include <sysdeps/unix/confstr.h>