N-Gage port: add changes from code reviews, overall cleanup (#5618)

* Add changes from code review by @ccawley2011, #5597, overall cleanup

* Update N-Gage README, minor cleanup and rephrasing

* Call SDL_SetMainReady() before calling SDL_main, return SDL_main instead of main

src/thread/ngage/SDL_syssem.cpp

@@ -20,7 +20,7 @@
 */
 #include "../../SDL_internal.h"
 
-/* An implementation of semaphores using mutexes and condition variables */
+/* An implementation of semaphores using the Symbian API. */
 
 #include <e32std.h>