Add custom error formatting (err_fmt config param)

    Add [libdefaults] err_fmt, a parameter for additional formatting of
    error messages.  %C gets substituted with the numeric error code.
    %M gets substituted with the original error message.

    For example:

        [libdefaults]
                err_fmt = %M (http://docs.foo.example/krb5/errors/%C)

    will produce errors like:

        Cannot find KDC for realm "FOO.EXAMPLE" (http://docs.foo.example/krb5/errors/-1765328230)