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.

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.