Existing D6 code was partially wrong already.  Tried to limit the fixes to new code as much as possible.

#308858 by smk-ka: Fixed user sessions sometimes not properly created in vBulletin due to a different vB behavior upon user login.

#285718 by smk-ka: Added protection against timing problems on heavy Drupal sites, where the redirector could try to register a user twice, resulting in broken user mappings.

#308860 by smk-ka: Added separate login/logout menu handlers to accommodate different session lifetimes.

#308857 by smk-ka: Fixed usernames containing non-latin1 characters not synced due to missing encoding.