plugins: imapsieve: Make sure responses are never sent.

Use the new SIEVE_EXECUTE_FLAG_SKIP_RESPONSES flag to prevent any unforseen
mishaps.

src/plugins/imapsieve/imap-sieve.c

@@ -597,7 +597,8 @@ static int imap_sieve_run_scripts
 		struct sieve_binary *sbin = scripts[i].binary;
 
 		cpflags = 0;
-		exflags = SIEVE_EXECUTE_FLAG_NO_ENVELOPE;
+		exflags = SIEVE_EXECUTE_FLAG_NO_ENVELOPE |
+			  SIEVE_EXECUTE_FLAG_SKIP_RESPONSES;
 
 		user_script = ( script == isrun->user_script );
 		last_script = script;
@@ -676,7 +677,8 @@ static int imap_sieve_run_scripts
 	}
 
 	/* Finish execution */
-	exflags = SIEVE_EXECUTE_FLAG_NO_ENVELOPE;
+	exflags = SIEVE_EXECUTE_FLAG_NO_ENVELOPE |
+		  SIEVE_EXECUTE_FLAG_SKIP_RESPONSES;
 	ehandler = (isrun->user_ehandler != NULL ?
 		isrun->user_ehandler : isieve->master_ehandler);
 	if ( compile_error == SIEVE_ERROR_TEMP_FAILURE ) {