malloc: allow attaching to external memory chunks

malloc: allow attaching to external memory chunks

In order to use external memory in multiple processes, we need to
attach to primary process's memseg lists, so add a new API to do
that. It is the responsibility of the user to ensure that memory
is accessible and that it has been previously added to the malloc
heap by another process.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>