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.

docs/README-migration.md

@@ -802,10 +802,9 @@ On Haiku OS, SDL no longer sets the current working directory to the executable'
 
 ```c
 {
-    char *path = SDL_GetBasePath();
+    const char *path = SDL_GetBasePath();
     if (path) {
         chdir(path);
-        SDL_free(path);
     }
 }
 ```