* accept_sec_context.c (krb5_gss_accept_sec_context),

* accept_sec_context.c (krb5_gss_accept_sec_context),
  init_sec_context.c (krb5_gss_init_sec_context),
  inq_cred.c (krb5_gss_inquire_cred): Call krb5_gss_validate_cred
  	to make sure the credential handle is still valid.

* val_cred.c (krb5_gss_validate_cred): New file which validates
	the credential to make sure it is valid, including
	checking to make sure the credentials cache still points
	at the same krb5 principal as it did before.

* accept_sec_context.c (krb5_gss_accept_sec_context): Return
	GSS_S_FAILURE if a non-NULL context handle is passed to it.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@10285 dc483132-0cff-0310-8789-dd5450dbe970