== DETAILS
- the free() method of the hid_driver_t interface needs its
parameter defined as const in order for the compiler to stop
complaining about losing const-ness.
- if a joypad list is created with <MAX_USERS slots in it, the
destroy() function will crash because it assumes there are MAX_USERS
entries.
To do this, the allocate function creates n+1 slots, and gives the
last slot a canary value that the destroy() method can then watch for
when iterating through the list.
