The refactoring in commit 471ab34032 wasn't complete enough: we
were checking the auth_len variable, but never actually setting it,
so it would never seem that authentication had been provided.
This commit also removes a bunch of unused variables from
rend_service_introduce, whose unusedness we hadn't noticed because
we were wiping them at the end of the function.
Fix for bug 8207; bugfix on 0.2.4.1-alpha.
