includes fix for nC-SA-2020-015.

See nextcloud/server#19976, the SA currently
has a typo - adressed in
nextcloud/security-advisories#21.

[19.09] nextcloud: 16.0.8 -> 16.0.9

The substitition in smtpd/parse.y isn't necessary anymore.
The hardcoded /usr/libexec/ has been replaced by a PATH_LIBEXEC #define,
which will be set properly by the build system.

(cherry picked from commit 9658850)

Fixes critical vulnerability:
  https://www.mail-archive.com/misc@opensmtpd.org/msg04850.html

(cherry picked from commit 7b9bd59)

(cherry picked from commit 77da495)

Release notes aren't available at this time [1] it is likely to be
related to a recent mail to oss-security (either [2] or [3]).

[1] https://www.mail-archive.com/misc@opensmtpd.org/msg04888.html
[2] https://www.openwall.com/lists/oss-security/2020/02/24/5
[3] https://www.openwall.com/lists/oss-security/2020/02/24/4

(cherry picked from commit 09725e5)

This reverts commit 4f69f2c.

We backported the latest opensmtpd version.

This reverts commit f5c74e6.

Already included in the opensmtpd version.

build fails against our local libressl version

opensmtpd: 6.4.2p1 -> 6.6.4p1 [backport 19.09]