On Windows, use pgwin32_waitforsinglesocket() instead of select() to wait for

On Windows, use pgwin32_waitforsinglesocket() instead of select() to wait for
input in the stats collector.  Our select() emulation is apparently buggy
for UDP sockets :-(.  This should resolve problems with stats collection
(and hence autovacuum) failing under more than minimal load.  Diagnosis
and patch by Magnus Hagander.

Patch probably needs to be back-ported to 8.1 and 8.0, but first let's
see if it makes the buildfarm happy...