libroot_debug: Merge guarded heap into libroot_debug.

The individual debug heap implementations are now exposed via a
structure of function pointers and a common frontend dispatches the
malloc and malloc_debug APIs through them.

The MALLOC_DEBUG environment variable can be used to select the guarded
heap by adding 'g', otherwise the debug heap is used as the default.
Consequently the separate libroot_guarded is not needed anymore and has
been removed.

To allow the use of environment variables this early, init_env_post_heap
has been added and the heap dependent atfork() moved there. This allowed
to fold the code of init_heap_post_env into init_heap so the former has
been removed.

headers/private/libroot/libroot_private.h

@@ -33,8 +33,8 @@ status_t __flatten_process_args(const char* const* args, int32 argCount,
 			char*** _flatArgs, size_t* _flatSize);
 void _call_atexit_hooks_for_range(addr_t start, addr_t size);
 void __init_env(const struct user_space_program_args *args);
+void __init_env_post_heap(void);
 status_t __init_heap(void);
-void __init_heap_post_env(void);
 void __heap_terminate_after(void);
 
 void __init_time(addr_t commPageTable);