nmbug.js: Third attempt at robustly loading the dialog polyfill

nmbug.js: Third attempt at robustly loading the dialog polyfill

It seems like onload is firing after the browser finishes *fetching*
the target, but before it finishes *parsing* the target.  That's
giving me ReferenceErrors for dialogPolyfill.  This new approach
attacks that directly, and just spins until dialogPolyfill has been
setup (with a bonus wait before using it, just for good measure).  Not
very pretty, but more robust ;).

I also simplified the check that avoids reloading the polyfills.
Instead of looking at the document's head, I just check for
dialogPolyfill.  If we've got native dialogs or dialogPolyfill, don't
bother (re)loading the polyfill scripts.