r13576: This is the beginnings of moving the SAM_ACCOUNT data structure

r13576: This is the beginnings of moving the SAM_ACCOUNT data structure
to make full use of the new talloc() interface.  Discussed with Volker
and Jeremy.

* remove the internal mem_ctx and simply use the talloc()
  structure as the context.
* replace the internal free_fn() with a talloc_destructor() function
* remove the unnecessary private nested structure
* rename SAM_ACCOUNT to 'struct samu' to indicate the current an
  upcoming changes.  Groups will most likely be replaced with a
  'struct samg' in the future.

Note that there are now passbd API changes.  And for the most
part, the wrapper functions remain the same.

While this code has been tested on tdb and ldap based Samba PDC's
as well as Samba member servers, there are probably still
some bugs.  The code also needs more testing under valgrind to
ensure it's not leaking memory.

But it's a start......