Feature #2438 add support for lmtp delivery to downstream servers

Feature #2438 add support for lmtp delivery to downstream servers

new feature lmtp support for simplifying setups with lmtp capable
downstream servers (e.g. dovecot) Postfix support lmtp out of the box
and can now deliver mails directly to internal mailbox servers
without one more smtp connection

extending the documentation for new lmtp option in
MailProxyConfiguration and MailProxyConfiguration:Transport

Signed-off-by: Julian Zehnter <pmg-devel@j-z.it>
Reviewed-By: Stoiko Ivanov <s.ivanov@proxmox.com>
Tested-By: Stoiko Ivanov <s.ivanov@proxmox.com>