Error message improvements

There's two commits here.

The first adds a [libdefaults] 'err_fmt' parameter (better naming suggestions welcomed) whose value is a string with %C (error code) and %M (error message) tokens, and is used in preparing a new error message out of what would have been krb5_get_error_message()'s output.

The second improves interpret_errno() and adds the ccache filename to the error message.  This includes a utility function, fcc_set_error(), that probably should be generic in kerrs.c.