Improve krb5_rd_req decryption failure errors

When krb5_rd_req cannot decrypt a ticket, try to produce the most
helpful diagnostic we can, and return an error code which corresponds
to the most applicable Kerberos protocol error.  Add a trace log
containing the error message for ticket decryption failures, in case
the application server does not log it.

Add new tests to cover krb5_rd_req error messages and adjust existing
tests to match the new messages.  Also adjust svc_auth_gssapi.c to
look for KRB5KRB_AP_ERR_NOT_US instead of KRB5KRB_AP_WRONG_PRINC.

ticket: 7232