trail: Avoid massive slowdown caused by pagetemplate hook when displaying dynamic...

trail: Avoid massive slowdown caused by pagetemplate hook when displaying dynamic cgi pages, which cannot use trail anyway.

This seemed to be due to the pagetemplate hook calling prerender. I've
observed this making it take *minutes* for the signin page to be displayed.
ltracing ikiwiki showed it was matching pagespecs a lot.

It may be that this is still a speed pain point when rendering pages, not
just for CGI. So more work may be needed here.