Provisioning /etc/dovecot/conf.d/*.conf is a pain on upgrade so we
consolidate that by reverting these files to the distro-provided ones
and shipping a single /etc/dovecot/conf.d/99-local.conf override
instead.

This provides better isolation opportunity as the service doesn't need
to run as ‘vmail’ user.  We use a dedicated system user instead, and
LDAP ACLs to limit its access to the strict minimum.

The new solution is also more robust to quoting/escaping, and doesn't
depend on ‘home=/home/mail/virtual/%d/%n’ (we might use $entryUUID
instead of %d/%n at some point to make user renaming simpler).

OTOH we no longer lists users that have been removed from LDAP but still
have a mailstore lingering around.  This is fair.

For `ssl_cipher_list` we pick the suggested value from
https://ssl-config.mozilla.org/#server=dovecot&version=2.3.9&config=intermediate&openssl=1.1.1d

At the moment it's equivalent (modulo order) to adding ‘EDH+AESGCM+aRSA’
to ‘EECDH+AESGCM:EECDH+CHACHA20!MEDIUM!LOW!EXP!aNULL!eNULL’.

This adds the following two ciphers:

  ECDHE-ECDSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH  Au=ECDSA Enc=CHACHA20/POLY1305(256) Mac=AEAD
  ECDHE-RSA-CHACHA20-POLY1305   TLSv1.2 Kx=ECDH  Au=RSA   Enc=CHACHA20/POLY1305(256) Mac=AEAD

And remove ‘ReadOnlyDirectories=/’ as it's implied by ‘ProtectSystem=strict’.