base.m4: Pass on authenticated sender properly in `smtp_local'.

  * Use the new `$acl_m_user' variable to identify the sender, if it's
    set; otherwise use the existing authenticated-sender.

  * Force setting `AUTH=...' to the next hop even though we haven't
    explicitly authenticated.  (Actually, we have, using a TLS client
    certificate, but that doesn't seem to count for pushing `AUTH=...'.)