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/.

internal/auth/auth_test.go

@@ -47,6 +47,7 @@ func TestDecodeResponse(t *testing.T) {
 		"", "\x00", "\x00\x00", "\x00\x00\x00", "\x00\x00\x00\x00",
 		"a\x00b", "a\x00b\x00c", "a@a\x00b@b\x00pass",
 		"\xffa@b\x00\xffa@b\x00pass",
+		"u@a/b\x00\x00pass", "u@[1.2.3.4]\x00\x00pass",
 	}
 	for _, c := range failedCases {
 		r := base64.StdEncoding.EncodeToString([]byte(c))