DSA-3828 dovecot - security update

It was discovered that the Dovecot email server is vulnerable to a
denial of service attack. When the dict passdb and userdb are used
for user authentication, the username sent by the IMAP/POP3 client is
sent through var_expand() to perform %variable expansion. Sending
specially crafted %variable fields could result in excessive memory
usage causing the process to crash (and restart).