BUG/MINOR: auth: free user groups on error paths in userlist_postinit()

BUG/MINOR: auth: free user groups on error paths in userlist_postinit()

In userlist_postinit(), when an error occurs (missing group, missing user, or
allocation failure), the function returned immediately without freeing the
auth_groups_list linked lists that were built for all users in the first loop.
Each user's curuser->u.groups pointed to these allocated nodes, which leaked
on every error path.

Fix by replacing direct returns with a goto to a centralized cleanup label
that frees all users' groups lists before returning the error. Also fix a
trailing double space in one error return statement while refactoring.

Note that the impact is very low since we're supposed to fail to boo after
such errors.