new feature: when delivering messages from the queue, make it possible to use a "transport"

the default transport is still just "direct delivery", where we connect to the
destination domain's MX servers.

other transports are:

- regular smtp without authentication, this is relaying to a smarthost.
- submission with authentication, e.g. to a third party email sending service.
- direct delivery, but with with connections going through a socks proxy. this
  can be helpful if your ip is blocked, you need to get email out, and you have
  another IP that isn't blocked.

keep in mind that for all of the above, appropriate SPF/DKIM settings have to
be configured. the "dnscheck" for a domain does a check for any SOCKS IP in the
SPF record. SPF for smtp/submission (ranges? includes?) and any DKIM
requirements cannot really be checked.

which transport is used can be configured through routes. routes can be set on
an account, a domain, or globally. the routes are evaluated in that order, with
the first match selecting the transport. these routes are evaluated for each
delivery attempt. common selection criteria are recipient domain and sender
domain, but also which delivery attempt this is. you could configured mox to
attempt sending through a 3rd party from the 4th attempt onwards.

routes and transports are optional. if no route matches, or an empty/zero
transport is selected, normal direct delivery is done.

we could already "submit" emails with 3rd party accounts with "sendmail". but
we now support more SASL authentication mechanisms with SMTP (not only PLAIN,
but also SCRAM-SHA-256, SCRAM-SHA-1 and CRAM-MD5), which sendmail now also
supports. sendmail will use the most secure mechanism supported by the server,
or the explicitly configured mechanism.

for issue #36 by dmikushin. also based on earlier discussion on hackernews.

doc.go

@@ -20,7 +20,7 @@ low-maintenance self-hosted email.
 	mox setadminpassword
 	mox loglevels [level [pkg]]
 	mox queue list
-	mox queue kick [-id id] [-todomain domain] [-recipient address]
+	mox queue kick [-id id] [-todomain domain] [-recipient address] [-transport transport]
 	mox queue drop [-id id] [-todomain domain] [-recipient address]
 	mox queue dump id
 	mox import maildir accountname mailboxname maildir
@@ -192,13 +192,19 @@ retry after 7.5 minutes, and doubling each time. Kicking messages sets their
 next scheduled attempt to now, it can cause delivery to fail earlier than
 without rescheduling.
 
-	usage: mox queue kick [-id id] [-todomain domain] [-recipient address]
+With the -transport flag, future delivery attempts are done using the specified
+transport. Transports can be configured in mox.conf, e.g. to submit to a remote
+queue over SMTP.
+
+	usage: mox queue kick [-id id] [-todomain domain] [-recipient address] [-transport transport]
 	  -id int
 	    	id of message in queue
 	  -recipient string
 	    	recipient email address
 	  -todomain string
 	    	destination domain of messages
+	  -transport string
+	    	transport to use for the next delivery
 
 # mox queue drop