Punt the tty-get/set-interrupt-chars primitives.

Punt the tty-get/set-interrupt-chars primitives.

Gtkio's signal forwarder breaks the assumptions in en/decode_interrupt
_handler.  Rather than fix them, I punt the TTY-GET-INTERRUPT-CHARS
and TTY-SET-INTERRUPTS-CHARS! primitives.  They were not used in The
Tree, not even by Edwin.