This routine formats and encrypts an AP_REP message, including
in it the data in the authentp portion
of auth_context, encrypted using the keyblock
portion of auth_context.
When successful, outbuf->length and outbuf->data are
filled in with the length of the AP_REQ message and allocated data
holding it. The outbuf->data argument
should be freed by the caller when it is no longer needed.
If the flags in auth_context indicate
that a sequence number should be used (either KRB5_AUTH_CONTEXT_DO_SEQUENCE
or KRB5_AUTH_CONTEXT_RET_SEQUENCE) and the local sequence number
in the auth_context is 0, a new number
will be generated with krb5_generate_seq_number.