pgkill for windows

Attached are 2 files needed to create a pgkill facility for Windows, and
a complementary patch for src/bin/Makefile

It is adapted from the facility on the Win32 page, but uses the builtin
signal definitions rather than using its own.

This facility is needed for shutting down postgres cleanly as part of
'make check', as we don't really use signals in Windows, we only emulate
them using our own scheme, so any native kill program doesn't have the
desired effect.

cheers

andrew