Fix some problems with unix signals: (1) Emacs uses SIGHUP to kill a

Fix some problems with unix signals: (1) Emacs uses SIGHUP to kill a
process when its buffer is killed, so treat SIGHUP differently if
under Emacs -- also don't print messages in this case as the buffer is
dead; (2) don't ask for core dump if signal received.

Also define new primitive `clear-interrupts!'; change
`get-next-interrupt-character' and `setup-timer-interrupt' so they
don't clear the interrupt bit.  It is now the responsibility of the
runtime system to clear the bit when it gets the interrupt.