Fix the createSession bug in spawnPipe
Fix the createSession bug in spawnPipe

Both the new XMonad.Core.xfork function and spawnPipe call createSession, calling
this function twice results in an error.

CaptainPatate/xmonad-contrib,0/xmonad-contrib,mathstuf/xmonad-contrib,0/xmonad-contrib,mathstuf/xmonad-contrib,ohlus/xmonad-contrib,0/xmonad-contrib,f1u77y/xmonad-contrib,ohlus/xmonad-contrib,f1u77y/xmonad-contrib,pjones/xmonad-test,xmonad/xmonad-contrib,xmonad/xmonad-contrib,ohlus/xmonad-contrib,CaptainPatate/xmonad-contrib,mathstuf/xmonad-contrib,CaptainPatate/xmonad-contrib