Add checks for existance of command to run in order to keep the
new quote handling capabilities of cmd.exe /C or /K. If the command
cannot be found, drop back to /S processing mode as per cmd.exe help.

dwStyle is updated only in WM_STYLECHANGED, which is not sent
when displaying and hiding scrollbars.

ddraw: Properly release the ddraw reference when the last interface is released in ddraw_surface_release_iface().