* add new home dir for Dovecot
I tried changing the mail dir, but this is a _very_ disruptive change,
so I took approach 3 on
<https://doc.dovecot.org/configuration_manual/home_directories_for_virtual_users/>,
whereby the home directory is now inside the mail directory.
The MDBOX/SDBOX formats are not touched by this change. The change
itself could be considered breaking though.
* adjust Sieve tests accordingly
* Update target/dovecot/10-mail.conf
* Update target/dovecot/auth-passwdfile.inc
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Co-authored-by: Casper <casperklein@users.noreply.github.com>
