Release candidate #3 for 6.5.5

This is a long overdue release candidate. It includes many fixes from the time of 6.5.5-rc2, here they are:

• Avoid lockups for IMAP synchronizations running with the
  "-1" command-line switch (X-Ryl669 boite.pour.spam@gmail.com)
• Dump stacktrace for all threads on SIGQUIT: ease debugging
  of threading and other issues
• SIGHUP is now handled as the termination notification rather than
  the signal to reread the configuration (Dmitrijs Ledkovs)
• Honor the timezone of emails (Tobias Thierer)
• Allow mbnames output to be sorted by a custom sort key by specifying
  a 'sort_keyfunc' function in the [mbnames] section of the config.
• Support SASL PLAIN authentication method. (Andreas Mack)
• Support transport-only tunnels that requre full IMAP authentication.
  (Steve Purcell)
• Make the list of authentication mechanisms to be configurable.
  (Andreas Mack)
• Allow to set message access and modification timestamps based
  on the "Date" header of the message itself. (Cyril Russo)
• "peritem" format string for [mbnames] got new expansion key
  "localfolders" that corresponds to the same parameter of the
  local repository for the account being processed.
• [regression] pass folder names to the foldersort function,
  revert the documented behaviour
• Fix handling of zero-sized IMAP data items (GitHub#15).
• Updated bundled imaplib2 to 2.35:
  • fix for Gmail sending a BYE response after reading >100 messages
    in a session;
  • includes fix for GitHub#15: patch was accepted upstream.
• Updated bundled imaplib2 to 2.36: it includes support for SSL
  version override that was integrated into our code before,
  no other changes.
• Fixed parsing of quoted strings in IMAP responses: strings like ""
  were treated as having " as the escaped quote, rather than treating
  it as the quoted escaped backslash (GitHub#53).

Release cycle for 6.5.5 should be really fast, only critical fixes should go into it, the rest will go into 6.5.6.

Happy testing!