filesystem: SDL_GetBasePath() now follows the SDL_GetStringRule.

It also now caches at the higher level, so the platform-specific bits don't
change their interface much.

A little code hygiene work was applied to some of the platform bits on top of
this.

Reference Issue #10229.

src/filesystem/unix/SDL_sysfilesystem.c

@@ -119,7 +119,7 @@ static char *search_path_for_binary(const char *bin)
 }
 #endif
 
-char *SDL_GetBasePath(void)
+char *SDL_SYS_GetBasePath(void)
 {
     char *retval = NULL;