[COMMITTED] Support for interruptible Poke programs

[Jose E. Marchesi]

Hi people!

I just pushed support for interrupting Poke programs, based on the work
Luca did in Jitter recently.

(poke) while (1) {}
^C
(poke)

Below is an excerpt from HACKING:

Signal Handling
~~~~~~~~~~~~~~~

When the PVM is about to execute a program, it installs a signal
handler that, at the moment, handles ``SIGINT``.  This signal handler
is defined in the ``late-c`` block in pvm.jitter, and records the
signal in the PVM internal state.

Then, when a ``sync`` instruction is executed, it checks the PVM
internal state and raises a ``PVM_E_SIGNAL`` exception if there is
some pending signal.  The default signal handler just ignores this
signal, but the user can also intercept it if desired.  The PKL
compiler generates ``sync`` instructions in strategic places, such as
before backward jumps in loops.

The ``exit`` PVM instruction clears the pending signals in the
internal PVM state.