pgsql: Add emulation of non-blocking sockets to the win32 socket/signal

Log Message:
-----------
Add emulation of non-blocking sockets to the win32 socket/signal layer,
and use this in pq_getbyte_if_available.

It's only a limited implementation which swithes the whole emulation layer
no non-blocking mode, but that's enough as long as non-blocking is only
used during a short period of time, and only one socket is accessed during
this time.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        pqcomm.c (r1.202 -> r1.203)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c?r1=1.202&r2=1.203)
    pgsql/src/backend/port/win32:
        socket.c (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/socket.c?r1=1.23&r2=1.24)
    pgsql/src/include/port:
        win32.h (r1.92 -> r1.93)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h?r1=1.92&r2=1.93)