Fix setsid in tty.open

Thanks to Leen Besselink for pointing this out.
Closes GH-815.

lib/tty_posix.js

@@ -45,7 +45,7 @@ exports.open = function(path, args) {
   child = spawn(path, args, {
     env: env,
     customFds: [masterFD, masterFD, masterFD],
-    setuid: true
+    setsid: true
   });
 
   return [stream, child];