Set msg_type when decoding FAST requests

An RFC 6113 KrbFastReq contains a padata sequence and a KDC-REQ-BODY,
neither of which contain the msg-type field found in a KDC-REQ.  So
when we decode the FAST request, the resulting krb5_kdc_req structure
has a msg_type of 0.  Copy msg_type from the outer body, since we make
use of it in further KDC processing.

ticket: 7605 (new)
target_version: 1.11.3
tags: pullup