Reject invalid characters in domains

Domains in addresses had very lax validation, which allowed them to
include characters like "/".

That could be exploited to trigger a daemon panic: a MAIL FROM
"x@evil/foo" could be accepted and, on failed delivery, a DSN queued.
Attempting to deliver the DSN would cause an MTA-STS cache lookup, which
would panic because the domain includes a "/" and crash the server.
Because the DSN would be queued, the server would crash again on
restarts, and only self-recover after give_up_send_after (20h by
default).

To fix this, we now explicitly validate the domain when normalizing it,
make MTA-STS cache not panic on invalid domains, and also make the SMTP
courier fail permanently on invalid domains.

Thanks to John JEAN <john@wargan.com> for finding and reporting this
problem. The detailed report can be found in
https://johnjean.io/advisories/chasquid-unauthenticated-remote-dos/.

test/t-12-minor_dialogs/bad_rcpt_to.cmy

@@ -26,7 +26,22 @@ c -> RCPT TO:<pepe>
 c <~ 501
 
 c -> RCPT TO:<a@xn--->
-c <- 501 5.1.2 Malformed destination domain (IDNA conversion failed)
+c <- 501 5.1.2 Malformed destination domain
+
+# Reconnect to avoid getting rejected due to too many errors.
+c close
+c tcp_connect localhost:1025
+c <~ 220
+c -> HELO localhost
+c <~ 250
+c -> MAIL FROM:<test@testy.com>
+c <~ 250
+
+c -> RCPT TO:<a@b/c>
+c <- 501 5.1.2 Malformed destination domain
+
+c -> RCPT TO:<a@[1.2.3.4]>
+c <- 501 5.1.2 Malformed destination domain
 
 # Reconnect to avoid getting rejected due to too many errors.
 c close