buffer: add property input_get_any_user_data in buffer (issue #2066)

This allows buffers to get any user input, including commands, that are sent to
the buffer callback instead of being executed on the buffer.

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

@@ -67,7 +67,7 @@ TEST_GROUP(CoreCommand)
             FAIL("Buffer not found");
         }
         record_start ();
-        input_data (buffer, command, NULL, 0);
+        input_data (buffer, command, NULL, 0, 0);
         record_stop ();
     }