invite 6.x-2.0-alpha1

#310775 by Barrett, sun: Fixed SQL error when no emails are left after filtering out registered users.
#323661 by tcconway: Removed "Track" from local task title "Track Invitations".
#214426 by barako: Updated French translation.

New features:
- #219266 by jcfiala, wojtha, smk-ka: Port to D6.
- Added administrator interface showing invite history.
- Moved the user-specific tracking page to the user profile.
- #162570: Added a resend button.
- #134997: Withdrawn invites are now flagged instead of being physically deleted. This gives administrators a complete history of who has been invited and who did or didn't join.
- #98765: Allowing multiple invites to the same e-mail address.
- #207419 by Andrew Young: Look up invitation for new users who didn't use invitation link.
- #214426 by Olivier Tavard: Added french translation.
- Refactored notification system to be less resource intensive.
- invite_stats() has been renamed to invite_count() and returns now a specific count.

Bugs fixed:
- #171765: Enhanced compatibility with LoginToboggan.
- #197198: Fixed misleading error message when no permissions have been set up.
- #196877: Prevent caching of user/register page (better approach).
- #229803 by will_in_wi: Restored capability for admins to create users when operating in "by invitation only" mode.
- #317552 by Stella: Use UTF8-safe string functions and other minor changes.