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/ipropd_slave.c

@@ -589,7 +589,7 @@ receive_everything(krb5_context context, int fd,
 	    if (ret)
 		krb5_err(context, IPROPD_RESTART_SLOW, ret, "hdb_store");
 
-	    hdb_free_entry(context, &entry);
+	    hdb_free_entry(context, mydb, &entry);
 	    krb5_data_free(&data);
 	} else if (opcode == NOW_YOU_HAVE)
 	    ;