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

test/t-12-minor_dialogs/auth_too_many_failures.cmy

@@ -10,9 +10,5 @@ c <~ 501
 c -> AUTH PLAIN something
 c <~ 501
 c -> AUTH PLAIN something
-c <~ 501
-c -> AUTH PLAIN something
-c <~ 501
-c -> AUTH PLAIN something
-c <~ 503 5.7.8 Too many attempts, go away
+c <~ 421 4.5.0 Too many errors, bye
 

test/t-12-minor_dialogs/bad_data.cmy

@@ -11,6 +11,13 @@ c <~ 250
 c -> DATA
 c <- 503 5.5.1 Sender not yet given
 
+# 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:<a@b>
 c <~ 250
 c -> RCPT TO: user@testserver

test/t-12-minor_dialogs/bad_mail_from.cmy

@@ -10,11 +10,25 @@ c <- 500 5.5.2 Unknown command
 c -> MAIL FROM:
 c <~ 500
 
+# 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:<pepe>
 c <~ 501
 
 c -> MAIL FROM:<a@xn--->
 c <- 501 5.1.8 Malformed sender domain (IDNA conversion failed)
 
+# 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:<aaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaXaaaa5aaaaX@bbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbXbbbb5bbbbX>
 c <- 501 5.1.7 Sender address too long

test/t-12-minor_dialogs/bad_rcpt_to.cmy

@@ -13,12 +13,30 @@ c <- 500 5.5.2 Unknown command
 c -> RCPT TO:
 c <~ 500
 
+# 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:<pepe>
 c <~ 501
 
 c -> RCPT TO:<a@xn--->
 c <- 501 5.1.2 Malformed destination domain (IDNA conversion failed)
 
+# 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:<henryⅣ@testserver>
 c <- 550 5.1.3 Destination address is invalid