I removed previous_handle_list in _execute_child since I noticed subprocess already clobbers the other attributes in startupinfo anyhow.

I figured there will be some discussion about how to pass the handle list, so here's my two cents:

* subprocess already exposes a bit of Windows specific flags like creationflags and STARTUPINFO.

* Windows doesn't really break it's API in backwards incompatible ways often (Heck it barely breaks it ever, which is why we have so many Ex functions and reserved parameters :P).

* The _winapi module tries to expose WinAPI functions as is. So I implemented this as an internal attribute on STARTUPINFO, in the first version, since I wasn't sure we want this exposed to users, but I still wanted to try and mimic the original WinAPI functions internally. The lpAttributeList is a change requested by eryksun that brings it even closer to WinAPI and exposes it for further extension with additional attributes.