DEBUG/MEDIUM: memory: add optional control pool memory operations

DEBUG/MEDIUM: memory: add optional control pool memory operations

When DEBUG_MEMORY_POOLS is used, we now use the link pointer at the end
of the pool to store a pointer to the pool, and to control it during
pool_free2() in order to serve four purposes :
  - at any instant we can know what pool an object was allocated from
    when examining memory, hence how we should possibly decode it ;

  - it serves to detect double free when they happen, as the pointer
    cannot be valid after the element is linked into the pool ;

  - it serves to detect if an element is released in the wrong pool ;

  - it serves as a canary, to detect if some buffers experienced an
    overflow before being release.

All these elements will definitely help better troubleshoot strange
situations, or at least confirm that certain conditions did not happen.