Remove SSLv3 support.

As far as I can see, SMTP, IMAP and POP clients are essentially
invulnerable to the attack du jour, since the server (or an impostor)
can't really make clients open new connections. It would be a strange
client that lets a server tell it to open thousands connections and
control the preauth commands.

But I see no real reason to support SSLv3 any more anyway, so this change
disable SSLv3 support.
Handle unicode append properly

This is a noop; it just accepts syntax and ignores it. the syntax modifier
is one I asked to have removed from the draft that became 6855 because its
job is indistinguishable from 'enable utf8=accept', and obviously I
thought it was removed.

Reported by Michael Slusarz.
Accept an empty list of return options

The empty list doesn't do anything, but is legal. Reported by Michael
Slusarz.
Accept some overlong SASL reponses

Base64 ignores random characters, so inserting extra spaces or quote
characters is harmless in the base64 stage. This change permits broken
clients that send "aaaa" instead of aaaa to work with digest-md5. This
concerns old versions of mailkit and possibly another client.