Remove / and \ at the start of shell.setDir() (#410)

Uses fs.combine to normalise the file path. This removes leading/trailing
slashes, as well as any redundant "../"s within the path.

src/main/resources/assets/computercraft/lua/rom/programs/shell.lua

@@ -194,7 +194,7 @@ function shell.setDir(_sDir)
     if not fs.isDir(_sDir) then
         error("Not a directory", 2)
     end
-    sDir = _sDir
+    sDir = fs.combine(_sDir, "")
 end
 
 function shell.path()