Multitap extensions to the controller interface

[andres-asm]

On many classic systems there was the option to have a multitap hooked up to a port. Now that option is irrelevant but since it fits with the retrospirit I was thinking the could be an addition to the retro_controller_description struct that can be used for such a goal.

struct retro_controller_description
{
   /* Human-readable description of the controller. Even if using a generic 
    * input device type, this can be set to the particular device type the 
    * core uses. */
   const char *desc;

   /* Device type passed to retro_set_controller_port_device(). If the device 
    * type is a sub-class of a generic input device type, use the 
    * RETRO_DEVICE_SUBCLASS macro to create an ID.
    *
    * E.g. RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 1). */
   unsigned id;

   /* Multitap device attached flag */
   bool multitap;
};

That multitap flag could be renamed to something more generic, but by doing this we could have a mutitap attached setting in the frontend and still use different controllers for actual input.

[andres-asm]

I've been checking cores, genplusgx for instance implements multitap on a per-port basis, but it's horrible since you have to add a controller for each combination:

#define RETRO_DEVICE_MDPAD_3B_WAYPLAY RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 3)
#define RETRO_DEVICE_MDPAD_6B_WAYPLAY RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 4)
#define RETRO_DEVICE_MDPAD_3B_TEAMPLAYER RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 5)
#define RETRO_DEVICE_MDPAD_6B_TEAMPLAYER RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 6)
#define RETRO_DEVICE_MSPAD_2B_MASTERTAP RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 7)

This is quite horrid imho.
When I asked ekeeke about moving this to core options he agreed but he said he was told it should be done like this: ekeeke/Genesis-Plus-GX#33

Other cores have this implemented as core options which seems cleaner but a bit out of place:

var.key = "beetle_psx_enable_multitap_port1";
if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
{
   if (strcmp(var.value, "enabled") == 0)
   setting_psx_multitap_port_1 = true;
   else if (strcmp(var.value, "disabled") == 0)
   setting_psx_multitap_port_1 = false;
}
var.key = "beetle_psx_enable_multitap_port2";
if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
{
   if (strcmp(var.value, "enabled") == 0)
   setting_psx_multitap_port_2 = true;
   else if (strcmp(var.value, "disabled") == 0)
   setting_psx_multitap_port_2 = false;
}

A flag and a corresponding toggle under input options would be neater and would help with standarization.