windows: use WSAGetLastError(), not errno
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.

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2