Some corrections to the SDL2.spec.in (thanks, Simone!). Fixes Bugzilla #2809.

Remove unnecessary parentheses and an unnecessary free() of a NULL pointer. (Thanks, Simon!) Fixes Bugzilla #2881.

Corrected documentation about triggers values in SDL_GameControllerGetAxis(). Fixes Bugzilla #2319.

We maintain the list of platforms on the wiki now. Also, I don't really want people emailing community members directly with problems, so I'm removing their email addresses. Fixes Bugzilla #2304.

Emscripten: Fixed receiving joystick events after failed init or subsystem quit. The callbacks used to receive the HTML events were not removed if the joystick subsystem initialization failed or if the joystick subsystem was quit. Also, the already connected joysticks were not deleted if the initialization failed later.

Removed redundant include statements from test programs. The needed header files are already included with SDL.h. Still including them in the test programs is confusing because it somehow suggests they would be needed.

X11: Don't create a window and GL context to look up the glX extension string. It's not necessary.

glX HasExtension(): check for NULL ext string first. It's slightly faster than failing later, after a strchr() call, since this will get called multiple times with a NULL string if the system totally fails elsewhere.

Emscripten: Removed unused internal function.

Emscripten: Changed return type of callback implementations from int to EM_BOOL. The prototypes are declared with EM_BOOL (which is currently an int) in html5.h.