win32 patch: processCancelRequest/reaper (+ various minor changes )

For application to HEAD, pending community review.

Briefly,

[all] Removed call to getppid in SendPostmasterSignal, replacing with a
PostmasterPid variable, which gets set (early) in PostmasterMain
- Q: Is this ok? Specifically, does anything call this function where
getppid would not be the postmaster?

[fork/exec] Implements processCancelRequest by keeping an array of
pid/cancel_key structs in shared mem

[fork/exec] Moves AttachSharedMemoryAndSemaphores call for backends into
SubPostmasterMain

[win32] Implements reaper/waitpid by keeping an arrays of children
pids,handles in postmaster local mem
- this item is largely untested, for reasons which should be
obvious, but appears sound

[win32/all] Added extern for pgpipe in Win32 case, and changed the second
pipe call (which seems to have been missed earlier) to pgpipe

[win32] #define'd ftruncate to chsize in the Win32 case

[win32] PG_USLEEP for Win32 has a misplaced paren. Fixed.

[win32] DLLIMPORT handling for MingW case

Cheers,
Claudio

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