Optional generation of Diffie-Hellman Parameters

[alxwr]

New since Dovecot 2.3.

From the mail log:

Jul 25 20:27:00 node2 dovecot: config: Warning: please set ssl_dh=</etc/dovecot/dh.pem                               
Jul 25 20:27:00 node2 dovecot: config: Warning: You can generate it with: dd if=/var/lib/dovecot/ssl-parameters.dat bs=1 skip=88 | openssl dhparam -inform der > /etc/dovecot/dh.pem

• https://wiki2.dovecot.org/Upgrading/2.3
• https://docs.iredmail.org/upgrade.dovecot.2.2-2.3.html

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Tested on FreeBSD 11.2 and Debian 10.