re-add explicit-powershell.exe-args

Variable is used implicitly by shell. See below for details

http://blogs.msdn.com/b/dotnetinterop/archive/2008/04/10/run-powershell-as-a-shell-within-emacs.aspx

This reverts commit 3484240d8289c6aaaa1c94ad1ee3d15b5b4c970e.