windows: uv_spawn shouldn't reject reparse points

This fixes an issue where uv_spawn would not try to run a reparse point,
and continue to scan the PATH instead. Effectively, it was impossible to
spawn a symlinked binary. This commit fixes that.

Also see #748

This is a backport of 495d1a0 to the v0.8 branch.
windows: use WSAGetLastError(), not errno

setsockopt() doesn't touch errno on failure. Use WSAGetLastError()
instead.

This is a back-port of commit 30a8b44 from the master branch.