SF patch #461413 (Gerhard Häring): Add STARTTLS feature to smtplib

SF patch #461413 (Gerhard Häring): Add STARTTLS feature to smtplib

   This patch adds the features from RFC 2487 (Secure SMTP
   over TLS) to the smtplib module:

   - A starttls() function
   - Wrapper classes that simulate enough of sockets and
     files for smtplib, but really wrap a SSLObject
   - reset the list of known SMTP extensions at each call
     of ehlo(). This should have been the case anyway.