Added thread-safe environment functions

Also marked the existing functions as unsafe, as they can cause crashes if used in multi-threaded applications.

As a bonus, since the new functions are hashtable based, hint environment lookups are much faster.

src/main/emscripten/SDL_sysmain_runapp.c

@@ -48,7 +48,7 @@ int SDL_RunApp(int argc, char* argv[], SDL_main_func mainFunction, void * reserv
                 _free(cvalue);
             }
         }
-    }, SDL_setenv);
+    }, SDL_setenv_unsafe);
 
     return mainFunction(argc, argv);
 }