Use library malloc for principal, policy entries

Use library malloc for principal, policy entries

Alter the KDB module contract to require that KDB modules use an
allocator compatible with the malloc() seen by libkrb5 and libkdb5.
Change krb5_db_alloc() and krb5_db_free() to provide access to this
allocator.  Remove free_principal, free_policy, alloc, and free from
the KDB interface and from all in-tree KDB modules.

ticket: 8414 (new)