New field codeno for smtp_envelope.

The value of this field denotes SMTP response code. Callbacks can
control what king of response will be sent to the client.
Callback return value influences smtp_parser.

If a callback function returns value different from zero, it is treated as success
and upon returning from smtp_parser_exec, number of parsed bytes is
returned. Otherwise zero is returned.

New function smtp_strresponse. This function translates smtp code number
into smtp response message which can be sent throught socket to client.