#504
Rework of #505

It is now apparent that page number validation cannot be done without
some participation from themes, at least themes that do "odd-ball"
things (such as having zenpage items on the index page.)

This change adds the ability for themes to participate in the decision
as to if page numbers are valid. Only default and perhaps zpmobile are
simple enough to not require participation.
Make URL page validation "opt-in" for themes

This is a change in behavior (all be it a good one) so theme designers
should not have to deal with it until at least 1.4.6
Merge remote-tracking branch 'origin/master' into 1.4.6

Conflicts:
	themes/stopdesign/functions.php