This is controlled by the new 'SSL_hostname' option.  The default value
of that option is the value of the 'host' option when it is hostname,
and the empty string (which disables SNI) when it is an IP literal.

This requires dovecot-imapd 2.2.31 or later.

Certificate generated with:

      $ openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -pkeyopt ec_param_enc:named_curve \
            -out tests/snippets/dovecot/dovecot.ecdsa.key
      $ openssl req -x509 -days 3650 -subj "/CN=InterIMAP test suite" \
            -key tests/snippets/dovecot/dovecot.ecdsa.key \
            -out tests/snippets/dovecot/dovecot.ecdsa.crt

Set "STARTTLS = NO" to ignore.  This is similar to CVE-2020-12398 and
CVE-2020-14093.

For background see https://gitlab.com/muttmua/mutt/-/issues/248 .

Cf. https://www.imapwiki.org/ClientImplementation/MailboxList#Hierarchy_separators
“Some clients cache the hierarchy separator forever. This has problems
if the server configuration is changed (e.g. server software changed).
Try to avoid this problem.”

This adds a dependency on Dovecot's LMTPd, which will bind to
to TCP port 10024 on the loopback interface.

UID EXPUNGE|FETCH|STORE commands are now split into multiple (sequential)
commands when their set representation exceeds 4096 bytes in size.  Without
splitting logic set representations could grow arbitrarily large, and
exceed the server's maximum command size.

This adds roundtrips which could be eliminated by pipelining, but it's
unlikely to make any difference in typical synchronization work.  While set
representations seem to remain small in practice, they might grow
significantly if many non-contiguous UIDs were flagged and/or expunged, and
later synchronized at once.

Furthermore, for MULTIAPPEND-capable servers, the number of messages is
limited to 128 per APPEND command (also subject to a combined literal size of
1MiB like before).

These numbers are currently not configurable.  They're intentionally lower
than Dovecot's default maximum command size (64k) in order to avoid a
deadlock situation after sending 8k-long commands under COMPRESS=DEFLATE:
https://dovecot.org/pipermail/dovecot/2019-November/117522.html .

The UNCHANGEDSINCE test from the CONDSTORE extension was incorrectly
placed after the flag list in UID STORE commands.  In practice this
meant the server didn't add the MODIFIED code when needed.

The server won't send an untagged FETCH command (and won't increase the
message's MODSEQ) if no change was made to the flag list.  A panic() was
incorrectly triggered in that case.

When the flag list was set (by another client) to a superset of the UID
STORE command currently processed, the extra flags were not synchronized.
Cf. RFC 7162 sec. 3.1.3 ex. 10.

An imapd is required as `doveadm exec imap` won't offer COMPRESS=DEFLATE
in its capability list.

SSL connections are accepted on TCP port 10993.  Also, fix STARTTLS
directive, broken since fba1c36…