Refactor so hook_init is not needed (replace with call to check if user is masquarding)

As title says, burdening bootstrap with the check seems unnecessary -- why does it need to be stored in a session and checked every page load? Wrapping it in a function allows it never be checked for certian checks/drush/etc.

Attaching patch, but for d6 version, so not putting it as needs review/work (assuming masquerade is get into 7 first than backport)