pgsql: On Windows, use pgwin32_waitforsinglesocket() instead of select()

Log Message:
-----------
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...

Modified Files:
--------------
    pgsql/src/backend/libpq:
        be-secure.c (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/be-secure.c.diff?r1=1.75&r2=1.76)
    pgsql/src/backend/port/win32:
        socket.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/socket.c.diff?r1=1.16&r2=1.17)
    pgsql/src/backend/postmaster:
        pgstat.c (r1.143 -> r1.144)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.143&r2=1.144)
    pgsql/src/include/port:
        win32.h (r1.71 -> r1.72)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.71&r2=1.72)