The auth_remote_user db table was not being updated correctly when
users were being moved from one auth method to another in bulk.
Fix adds in the correct lines to that table. Code similar to what
is used in admin/users/edit.php for single user update.

Moved some things around to improve performance and fixed a mistake
Used a better way to check authname

Change-Id: Ieb2d453bc06646444aadaff92ea5b42af4246411
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>