x86/paging: Enforce PG_external == PG_translate == PG_refcounts

x86/paging: Enforce PG_external == PG_translate == PG_refcounts

Setting PG_refcounts but not PG_translate is not useful.

While adjusting this, make a few other improvements.

 * Have paging_enable() unilaterally reject any unknown modes.
 * Drop the or'ing of PG_{HAP,SH}_enable.  The underlying functions already do
   this.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>