Try decrypting using session key if subkey fails in tgs rep handling

Try decrypting using session key if subkey fails in tgs rep handling

Heimdal at least up through 1.2 incorrectly encrypts the TGS response
in the session key not the subkey when a subkey is supplied.  See RFC
4120 page 35.  Work around this by trying decryption using the session
key after the subkey fails.

* decode_kdc_rep.c: rename to krb5int_decode_tgs_rep; only used for
  TGS and now needs to take keyusage
* gc_via_tkt: pass in session key and appropriate usage if subkey
  fails.

Note that the dead code to process AS responses in decode_kdc_rep is
not removed by this commit.  That will be removed as FAST TGS client
support is integrated post 1.7.

ticket: 6484
Tags: pullup
Target_Version: 1.7

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@22325 dc483132-0cff-0310-8789-dd5450dbe970