joystick: Add an axis to allow strafing.

Modern gamepads typically have 2-3 D-pads and joysticks. This means
that it's desirable to be able to use one joystick for turning and
another for strafing. Add another axis in addition to the current X
and y axes that performs strafe movement.
joystick: Add joystick button to toggle menu.

When using a joystick or gamepad it's nice to be able to bring up the
menu without having to reach for the keyboard. This makes modern
gamepads more useful/usable.
setup: Change labelling to clarify gamepad support.

Vanilla Doom's setup.exe only made reference to "joysticks" as back
then gamepads were relatively uncommon for PCs and ports for game
controllers canonically known as "joystick ports". Nowadays it's far
more likely (and ergonomic) that the player will be using a gamepad
than a joystick. Change the labelling to refer to "Gamepad/Joystick"
or "controller" instead.