Update to Dovecot 2.1.7

* LDAP: Compatibility fix for v2.0: ldap: If attributes contain
  ldapAttr=key=template%$ and ldapAttr doesn't exist, skip the key
  instead of using "template" value with empty %$ part for the key.

+ pop3: Added pop3_uidl_duplicates setting for changing the behavior
  for duplicate UIDLs.
+ director: Added "doveadm director ring remove" command.
- director: Don't crash with quickly disconnecting incoming director
  connections.
- mdbox: If mail was originally saved to non-INBOX, and namespace
  prefix is non-empty, don't assert-crash when rebuilding indexes.
- sdbox: Don't use more fds than necessary when copying mails.
- auth: Fixed crash with DIGEST-MD5 when attempting to do master user
  login without master passdbs.
- Several fixes to mail_shared_explicit_inbox=no
- imapc: Use imapc_list_prefix also for listing subscriptions.

(ghen)