win32: Wrap bg cmd in quotes

If a command both starts and ends in quotes (e.g. `"sumatrapdf" "file.pdf"`), these outer quotes seem to be removed from the command when run under `!start /b`, creating an invalid command (the example becomes `sumatrapdf" "file.pdf`).

Wrapping the command in quotes seems to work as a fix, so that there are always outer quotes to be removed.