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.

kdc/mit_dump.c

@@ -209,7 +209,7 @@ mit_prop_dump(void *arg, const char *file)
             continue;
         }
 	ret = v5_prop(pd->context, NULL, &ent, arg);
-        hdb_free_entry(pd->context, &ent);
+        hdb_free_entry(pd->context, NULL, &ent); /* XXX */
         if (ret) break;
     }