This routine formats a KRB_AP_REQ message
The principal of the server to receive the message is specified
by hostname and service.
If credentials are not present in the credentials cache ccache for
this server, the TGS request with default arguments is used in an
attempt to obtain such credentials, and they are stored in ccache.
The checksum method to be used is as specified in auth_context.
The outbuf argument should point
to an existing krb5_data structure. outbuf->length and outbuf->data will
be filled in on success, and the latter should be freed by the caller
when it is no longer needed; if an error is returned, however, no
storage is allocated and outbuf->data does
not need to be freed.