Use DEFTAGGEDTYPE for FAST choice types

Use DEFTAGGEDTYPE for FAST choice types

pa_fx_fast_request and pa_fx_fast_reply are defined in RFC 6113 as
extensible choice types with only one choice, which means they encode
as another type with a [0] tag wrapper.  Use DEFTAGGEDTYPE to add
this wrapper instead of DEFFIELDTYPE/FIELDOF_ENCODEAS, and add a
comment describing why.

After this change, all uses of DEFFIELDYPE are for dataptr/lenptr
types or just simple offset transformations on the base pointer.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25635 dc483132-0cff-0310-8789-dd5450dbe970