input: add input test mode

In input test mode, key bindings won't be executed, but are shown on the
OSD.  The OSD includes various information, such as the name of the key,
the command itself, whether it's builtin, and the config file location
it was defined.

The input test mode can be enabled with "--input=test". No effort is
spent trying to react to key bindings that normally exit the player;
they are treated just like any other binding.

input/input.h

@@ -130,6 +130,7 @@ typedef struct mp_cmd {
     int nargs;
     int pausing;
     enum mp_on_osd on_osd;
+    bstr original;
     struct mp_cmd *queue_next;
 } mp_cmd_t;