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/dummy/SDL_sysfilesystem.c

@@ -25,7 +25,7 @@
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 /* System dependent filesystem routines                                */
 
-char *SDL_GetBasePath(void)
+char *SDL_SYS_GetBasePath(void)
 {
     SDL_Unsupported();
     return NULL;