Set ctrl-h to be recognized as backspace.

On my terminal (default xterm on Slackware), ctrl-h (ASCII 8) is used for backspace.  This merely caused a beep in arvo, so I set it to be recognized as a backspace.  I did this on the c side because I'm not familiar enough with Hoon to change urb/zod/arvo/dill.hoon, but I think one could implement this after line 161 of that file (actions bound to control characters).  I don't really know if this will have any unintended side-effects, but it seems to be working fine for me.