pgsql: On win32, retry reading when WSARecv returns WSAEWOULDBLOCK.

Log Message:
-----------
On win32, retry reading when WSARecv returns WSAEWOULDBLOCK. There seem
to be cases when at least Windows 2000 can do this even though select
just indicated that the socket is readable.

Per report and analysis from Cyril VELTER.

Modified Files:
--------------
pgsql/src/backend/port/win32:
socket.c (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/socket.c.diff?r1=1.17&r2=1.18)