smtpsrv: Close the connection after 3 errors (lowering from 10)

Today, we close the connection after 10 errors. While this is fine for
normal use, it is unnecessarily large.

Lowering it to 3 helps with defense-in-depth for cross-protocol attacks
(e.g. https://alpaca-attack.com/), while still being large enough for
useful troubleshooting and normal operation.

As part of this change, we also remove the AUTH-specific failures limit,
because they're covered by the connection limit.

test/t-12-minor_dialogs/auth_multi_dialog.cmy

@@ -13,6 +13,13 @@ c <~ 334
 c -> dXNlckB0ZXN0c2VydmVyAHlalala==
 c <~ 501 5.5.2 Error decoding AUTH response
 
+# Reconnect to avoid getting rejected due to too many errors.
+c close
+c tls_connect localhost:1465
+c <~ 220
+c -> EHLO localhost
+c <... 250 HELP
+
 c -> AUTH PLAIN
 c <~ 334
 c -> dXNlckB0ZXN0c2VydmVyAHVzZXJAdGVzdHNlcnZlcgB3cm9uZ3Bhc3N3b3Jk