In with the scabs, out with the tr_msg union!

In with the scabs, out with the tr_msg union!

The tr_msg union lead to a number of security issues because the code
tended to check to see if msg->msg_struct_name was non-null.  However
it was always non-null because the pointer was shared among all the
union members.  Instead, use accessors for everything.
LP: #1333734