Reviewed locking in mmalloc:

* Correctly initialize and destroy the mdp mutex.
* Simplify locking logic by pushing it up in {m,c,re}alloc/free
  in "mm_legacy.c".
* Lock the default_mdp mutex around fork calls
  (introduce xbt_os_thread_atfork for this purpose).
* Increase HEAP_OFFSET to avoid strange errors with valgrind
  (got unexplained memory corruption in tesh with several threads).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@8506 48e7efb5-ca39-0410-a469-dd3cf9ba447f