added a cd() builtin to the shell, for changing working directory

exec/shell.lua

@@ -3,6 +3,25 @@ local shenv = {}
 function shenv.quit()
  os.setenv("run",nil)
 end
+function shenv.cd(p)
+ if p:sub(1,1) == "/" then
+  if fs.list(p) then
+   os.setenv("PWD",p)
+  else
+   print("no such directory: "..p)
+  end
+ else
+  local np = {}
+  for k,v in pairs(fs.segments(os.getenv("PWD").."/"..p)) do
+   if v == ".." then
+    np[#np] = nil
+   else
+    np[#np+1] = v
+   end
+  end
+  os.setenv("PWD","/"..table.concat(np,"/"))
+ end
+end
 setmetatable(shenv,{__index=function(_,k)
  if _G[k] then
   return _G[k]