Check shell in Windows before assuming cmd.exe

From https://github.com/kien/ctrlp.vim/pull/598:

> In Windows, some users set the Cygwin shell as their Vim shell, consider
> this before rewriting cd calls. Checking if shellcmdflag contains a
> backslash should catch all true windows shells.
> 
> Without this fix, user list commands are broken with a cygwin shell.