Add mipsel and mips64el to the list of architectures where we set
-funwind-tables, as this seems to be necessary to support stacktrace
generation on these architectures.

Fix FTBFS on s390x

Try to fix FTBFS on several architectures.

991323: CVE-2020-28200
983510: dovecot-core: repeated user enumeration fails with sssd backend (and probably others)

The /etc/dovecot/README hasn't existed in multiple debian stable releases; this
directive is thus no longer needed and should be removed as it's now triggering
lintian errors:

E: dovecot-core: conffile-is-not-in-package remove-on-upgrade /etc/dovecot/README
E: dovecot-core: non-etc-file-marked-as-conffile remove-on-upgrade /etc/dovecot/README
E: dovecot-core: relative-conffile remove-on-upgrade /etc/dovecot/README

Document addition of indexer-Fix-crash-if-client-disconnects-while-it-s-w.patch

See https://dovecot.org/pipermail/dovecot/2021-August/122786.html and
https://github.com/dovecot/core/commit/97367ef61784a364ccf615cb816be6492329050d

Document addition of debian/patches/indexer-crash.patch

Refresh git commit references

Import patch from https://dovecot.org/pipermail/dovecot/2021-August/122939.html

Update to upstream version '2.3.16+dfsg1'
with Debian dir f5ebb59c21a63d6383e6f80a1c0613fec2e39ec0

- ssl-cert-location
  Adjust description, we are not disabling SSL by default

- default-mail_location
  Drop note about non existing chgrp-error.patch

- fix-compiler-warnings
  Add forwarded header

- exampldir
  Drop patch
  It is already inactive cause it changes Makefile.in files, which get
  overridden by autoreconf.
  Also /etc/dovecot/README is not installed anymore and the
  configuration examples are currently installed into /usr/share/dovecot
  and /usr/share/dovecot/conf.d

- CVE-2021-29157: Path traversal issue allowing an attacker with
  access to the local filesystem can trick OAuth2 authentication into
  using an HS256 validation key from an attacker-controlled location
- CVE-2021-33515: Sensitive information could be redirected to an
  attacker-controlled address because of a STARTTLS command injection

The test suite does not pass reprotest:

test-buffer-istream.c:54: Assert failed: buffer_append_full_file(result, TEST_FILENAME, SIZE_MAX, &error) == BUFFER_APPEND_READ_ERROR
test-buffer-istream.c:56: Assert failed: error != NULL && *error != '\0'
buffer_append_full_file .............................................. : FAILED

But generally dovecot builds reproducible, see
https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/dovecot.html