Fix handling of RET_SEQUENCE flag in mk_priv/mk_ncred

Fix handling of RET_SEQUENCE flag in mk_priv/mk_ncred

Regularize the handling of KRB5_AUTH_CONTEXT_RET_SEQUENCE in
krb5_mk_safe, krb5_mk_priv, and krb5_mk_ncred, using krb5_mk_safe as
a baseline.  RET_SEQUENCE now implies DO_SEQUENCE for all three
functions, the sequence number is always incremented if it is used,
and outdata->seq is always set if RET_SEQUENCE is passed.

Note that in the corresponding rd_ functions, RET_SEQUENCE and
DO_SEQUENCE are independent flags, which is not consistent with the
above.  This compromise is intended to preserve compatibility with
any working code which might exist using the RET_SEQUENCE flag.

ticket: 6478

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@22288 dc483132-0cff-0310-8789-dd5450dbe970