libpq win32-native errno fix

i've spotted a following problem using DBD::Pg under win32. winsock
functions do not set errno, so in some normal conditions are treated as
fatal errors. e.g. fetching large tuples fails, as at some point recv()
returns EWOULDBLOCK. here's a patch, which replaces errno with
WSAGetLastError(). i've tried to to affect non-win32 code.

/Dmitry