Win32 signals patch #2

Hello!

Here's step #2 in win32 signals handling, containing the following:

1) Per discussion with Bruce, reverts the change from kill() to pqkill()
on all platforms. Instead, #define away kill() to pqkill() in
port/win32.h, and just use kill() directly on unix platforms. Similar
changes for pqsigsetmask, except they are all local to pqsignal.h.

2) Implements pqselect() in similar ways. This function appears just as
select() to the user. Internally, it runs a loop where it polls for
signals once / second. This allows the backend to actually shut down,
since we can deliver signals to both postmaster and statistics
processes. Ordinary backends cannot yet be shut down, since they don't
use select, but a blocking recv() instead. I'll get to those soon...

//Magnus