Prior to this change, a lot of code uses the number of an emulated joystick
device to correlate various arrays and perform other case distinctions.
This change here introduces a more object-oriented approach: A single struct
contains all the information about an emulated stick, and functions to get
state can use that struct without worrying about the index.

The change removes the js_state structure.  Since SDL will already have the
current state cached, adding a second caching layer just adds complexity and
data duplication.  The get_platform_PORT and get_platform_TRIG functions are
also removed: since PLATFORM_PORT and PLATFORM_TRIG request state for just
one or two devices at a time, computing state for all of them feels wasteful
and confusing.

This change fixes a minor bug related to a failed SDL_JoystickOpen call.  In
that case, the variables i and joysticks_found inside Init_SDL_Joysticks
might mismatch.  That can lead to an incorrect assignment of the number of
buttons.  Apart from this, there should be no user-observable changes
introduced by this commit.

The rest of this commit message will look at some finer nuances of how
the old functionality gets modeled in the new code.

* The variables kbd_joy_#_enabled and swap_joysticks now require a
  call to update_kbd_sticks to take effect, since it's the stick_dev.kbd
  pointer that really matters here.  That's the reason the ui must now use
  functions instead of variables to access these.  Note that swap_joysticks
  only applies to keybord-emulated sticks, both before and after this
  change.
* The arguments to Init_SDL_Joysticks were without effect so got dropped.
* The four separate kbhits checks were replaced by a loop. The index
  in the configuration array corresponds to the bit for that direction.
  Alternative would have been to use a struct instead of an array, and to
  keep four separate id conditions instead of the loop.
* The presence of an LPT joystick overrides the SDL joystick of the
  same index.  This used to be due to an if-else construct in
  get_platform_PORT and get_platform_TRIG but is now achieved by
  Init_SDL_Joysticks clearing the SDL pointer on those devices.
* For the OSK, joysticks_found != 0 translates to osk_stick != NULL.
* Instead of relying on locally cached state, the on-screen keyboard
  now queries buttons individually at the time they are needed.  The
  joystick direction (port) state is also kept local.

This new flag allows users to emulate a larger number of joysticks by not
combining multiple physical devices into a single emulated joystick.

For example this allows using keyboard to emulate one or two joysticks, and
to use these in conjunction with physical SDL joysticks / controllers in
order to play with more people than there are SDL joysticks.
The same logic gets applied to LPT joysticks, too: if the flag is present,
these come after joysticks from keyboard emulation.

Furthermore, the logic also gets applied to put SDL joysticks after LPT
joystics even when the command line flag isn't set.  Since LPT joysticks
take precedence over SDL joysticks on the same emulated device, this takes a
device that was completely ignored before and makes it useful now.

The assignments are fixed at program start-up, after config file and command
line flags have been taken into account.  If the UI is used to enable or
disable a keyboard-emulated joystick, this can cause an emulated joystick to
be controlled by multiple physical devices, or by none.