Add emulation of non-blocking sockets to the win32 socket/signal layer,

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.