api: don't split on newline by default in functions command and command_options when input_multiline is set to 0

The API functions `command` and `command_options` (when `split_newline` = 0,
which is the default value) don't split on newline and then the first line is
executed and the subsequent lines (after "\n") are ignored.

There are no changes when the input has multiple lines filled by the user: the
split is done and multiple commands are executed (for example if the user is
pasting multiple commands to execute).

tests/unit/core/test-core-command.cpp

@@ -70,7 +70,7 @@ TEST_GROUP(CoreCommand)
             FAIL("Buffer not found");
         }
         record_start ();
-        input_data (buffer, command, NULL);
+        input_data (buffer, command, NULL, 0);
         record_stop ();
     }