hdb: decorate HDB_entry with context member

Decorate HDB_entry with context and move free_entry callback into HDB structure
itself. Requires updating hdb_free_entry() signature to include HDB parameter.
A follow-up commit will consolidate hdb_entry_ex (which has a single hdb_entry
member) into hdb_entry.

lib/kadm5/randkey_s.c

@@ -190,7 +190,7 @@ kadm5_s_randkey_principal(void *server_handle,
 	*n_keys = 0;
     }
  out3:
-    hdb_free_entry(context->context, &ent);
+    hdb_free_entry(context->context, context->db, &ent);
  out2:
     (void) kadm5_log_end(context);
  out: