Win32 signals & sockets

Finally, here is the patch:

* Create the signal pipe in the postmaster and then inherit it into the
child
* Duplicate sockets using WSADuplicateSocket/WSASocket to get around
buggy LSP implementations. From my testing this does not solve *all*
problems, but it does solve a lot of them.
* Change pgstat so it signals the collector when the bufferer
terminates. Because some LSPs are still broken and process termination
just won't work when the pipe goes away.
* Clean up handling of the backend parameter file some. There was no
error checking at all in large parts of it before, which bit me during
testing, so I figured I might as well take a stab at that. This moves it
all into a structure and writes that one - which is also preparation for
the move of the parameter structure away from a file and into shared
memory.

If this is accepted I also plan to do a patch to split out the forkexec
code into a separate file and try to clean up the dependencies a bit
further. It'd be nice if I could get that into 8.0.0 (which would
probably mean this beta, since it seems to be the last one), but it's
not critical.
I'd also like to take a look at moving the parameter file into shared
memory at least on win32, since it gives thefilesystem quite a bit of
trashing. I think that can be done pretty easily, but I'm not sure if
there'll be tinme for that in 8.0.0 either.

//Magnus