xv6sh-linux port

This program is a fork of the xv6-public's shell modified to work on linux. 
xv6sh is a very tiny, minimal, non bloated shell which is good enough as long
as youre not doing complicated shell scripting. I would not link this to /bin/sh
but i can see myself using this shell on a minimal environment. 

to build run: gcc xv6sh.c