If -debug_dap flag is given, xsystem35 speaks the Debug Adapter Protocol
(https://microsoft.github.io/debug-adapter-protocol/) via stdin/stdout.
This enables xsystem35 to work as a backend of VS Code's Debugger (and
possibly other debuggers that support DAP).
This removes X11 specific code, making SDL2 a hard dependency.
X-based environments are continue to be supported via SDL2.
This also removes joystick_linux.c which was only used if SDL2 was not
available.