Change keytab names to reflect smtp/ principal

In order to get kerberized SMTP by Postfix backending onto Dovecot's SASL
implementation, Dovecot needs to have access to an smtp/olinda.dehnerts.com key
(as well as an imap/olinda.dehnerts.com key for the IMAP). Sticking
smtp/olinda.dehnerts.com in a file named imap.keytab seemed silly, so I renamed
that file to olinda.keytab and added the smtp/ key. Getting and using the new
principal (and reconfiguring the MUA's like Thunderbird) turns out to be the
only config change required.