Log Message:
Update squirrelmail to 1.4.23pre14605, latest snapshot.
PHP 7.0 support should be improved, too.

  - Added new "smtp_helo_override" hook; allows plugins to override
    the HELO host sent to the SMTP server when sending messages
  - Added STARTTLS support for both IMAP and SMTP connections
  - Added PDO support for database connections, so no external
    database module needs to be installed