Win32 signals cleanup. Patch by Magnus Hagander, with input from Claudio

Win32 signals cleanup. Patch by Magnus Hagander, with input from Claudio
Natoli and Bruce Momjian (and some cosmetic fixes from Neil Conway).
Changes:

    - remove duplicate signal definitions from pqsignal.h

    - replace pqkill() with kill() and redefine kill() in Win32

    - use ereport() in place of fprintf() in some error handling in
      pqsignal.c

    - export pg_queue_signal() and make use of it where necessary

    - add a console control handler for Ctrl-C and similar handling
      on Win32

    - do WaitForSingleObjectEx() in CHECK_FOR_INTERRUPTS() on Win32;
      query cancelling should now work on Win32

    - various other fixes and cleanups