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