This fixes CVE_2020-24386, CVE-2020-25725 and a bunch of regular bugs
[1].

* CVE-2020-24386: Specially crafted command can cause IMAP hibernate to
	  allow logged in user to access other people's emails and filesystem
	  information.

* CVE-2020-25275: Mail delivery / parsing crashed when the 10 000th MIME part was
  message/rfc822 (or if parent was multipart/digest). This happened
  due to earlier MIME parsing changes for CVE-2020-12100.

[1] https://raw.githubusercontent.com/dovecot/core/2.3.13/NEWS

(cherry picked from commit 58c7d3f)

While we already had some test we might as well add the test for that
exact package to the tests attribute set. After all that should be what
(primarily) tests dovecot.

(cherry picked from commit 4fee20e)

This updates to the latest version. According to the changelog 0.5.12
was skipped. The changes in this release are required to be compatible
with the latest dovecot release.

Changes:
  - duplicate: The test was handled badly in a multiscript (sieve_before,
    sieve_after) scenario in which an earlier script in the sequence with
    a duplicate test succeeded, while a later script caused a runtime
    failure. In that case, the message is recorded for duplicate tracking,
    while the message may not actually have been delivered in the end.
  - editheader: Sieve interpreter entered infinite loop at startup when
    the "editheader" configuration listed an invalid header name. This
    problem can only be triggered by the administrator.
  - relational: The Sieve relational extension can cause a segfault at
    compile time. This is triggered by invalid script syntax. The segfault
    happens when this match type is the last argument of the test command.
    This situation is not possible in a valid script; positional arguments
    are normally present after that, which would prevent the segfault.
  - sieve: For some Sieve commands the provided mailbox name is not
    properly checked for UTF-8 validity, which can cause assert crashes at
    runtime when an invalid mailbox name is encountered. This can be
    caused by the user by writing a bad Sieve script involving the
    affected commands ("mailboxexists", "specialuse_exists").
    This can be triggered by the remote sender only when the user has
    written a Sieve script that passes message content to one of the
    affected commands.
  - sieve: Large sequences of 8-bit octets passed to certain Sieve
    commands that create or modify message headers that allow UTF-8 text
    (vacation, notify and addheader) can cause the delivery or IMAP
    process (when IMAPSieve is used) to enter a memory-consuming
    semi-infinite loop that ends when the process exceeds its memory
    limits. Logged in users can cause these hangs only for their own
    processes.
(cherry picked from commit bedb352)

This plugin is used commonly enough that we should ensure it still
builds (and dovecot works) after loading it.

This is not yet perfect as we aren't testing any of it's functionality
but at least we ensure that dovecot continues to do the regular job.

(cherry picked from commit 653f805)