chasquid: Detect email loops

This patch implements some measures against email loops, such as keeping
a limit on the lenght of an address, and rejecting email that has too
many Received headers.

It's not perfect (a server could be actively removing Received headers),
but it should cover the normal accidents and misconfigurations.

chasquid_test.go

@@ -108,7 +108,7 @@ func sendEmailWithAuth(tb testing.TB, c *smtp.Client, auth smtp.Auth) {
 		tb.Fatalf("Data: %v", err)
 	}
 
-	msg := []byte("Hi! This is an email\n")
+	msg := []byte("Subject: Hi!\n\n This is an email\n")
 	if _, err = w.Write(msg); err != nil {
 		tb.Errorf("Data write: %v", err)
 	}