os-posix: reorder parent notification for -daemonize

Put "success" parent reporting in os_setup_post() to after
all other initializers which may also fail, to the very end,
so more possible failure cases are reported properly to the
calling process.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
