spamassassin: Increase --max-children to 10

This used to be 5, but occasionally a spike in incoming mails would run
out of room to spawn new spamd instances. This wasn't a big problem by
itself, since up to 128 new connections are queued, but allowing a few
more instances might make things more smooth (though these spamd
instances fight for the same CPU resources, so it's probably only a
small gain).

Furthermore, increasing max-children prevents warnings about it from
showing up in the logs all the time, but still allow the warning to
attract attention when there is a really big load spike or other
problem.from showing up in the logs all the time, but still allow the
warning to attract attention when there is a really big load spike or
other problem.
dovecot: Document workaround for sievec bug
dovecot: Use the uniqueIdentifier as internal "username"

Previously, the primary mail address was used, but using the
uniqueIdentifier allows delivering mail to accounts without any email
address. Additionally, it saves an LDAP lookup in the userdb, since the
homedir can now be constructed using a template.

The username passed to lda must now be the uniqueIdentifier, so this
needs changes in Exim. On the IMAP side, things didn't change, since the
passdb still looks up users using their primary mail address.
dovecot: Switch to mail.tikatika.nl SSL certificates
dovecot: Autcreate and subscribe mailboxes during delivery

When mail is delivered to a non-existing mailbox (e.g. through a Sieve
script), it is automatically created. Additionally, the new mailbox is
also added to the user's subscription list, so it shows up in roundcube
and other mailclients that only show subscribed folders.
exim: Merge ADDR and Q_ADDR variables

These really were identical, so might as well use just one.
exim: Fix indent
exim: Fix ldap_group_member router

Due to an upgrade of exim, the way multiple "member" attributes were
returned has changed and this accounts for that.

Previously, these were joined using ", " and any commas in the member
dns were untouched, requiring (and allowing) a hack replacing ", " with
newlines and then splitting on newlines to access the individual member
dns.

In more recent versions, the multiple dns are separated just by ",", but
any commas inside the dns are doubled. This allows using exim's
alternate separator syntax {<, ...} directly to split on commas, while
at the same time undoubling the commas.
exim: Pass the uniqueIdentifier to dovecot-deliver

Previously, the primary mail was used, but dovecot configuration was
changed to accept the uniqueIdentifier instead.
exim: Improve debug output