Set CREATE_NO_WINDOW in tor_spawn_background.

This flag prevents the creation of a console window popup on Windows. We
need it for pluggable transport executables--otherwise you get blank
console windows when you launch the 3.x browser bundle with transports
enabled.

http://msdn.microsoft.com/en-us/library/ms684863.aspx#CREATE_NO_WINDOW

The browser bundles that used Vidalia used to set this flag when
launching tor itself; it was apparently inherited by the pluggable
transports launched by tor. In the 3.x bundles, tor is launched by some
JavaScript code, which doesn't have the ability to set CREATE_NO_WINDOW.
tor itself is now being compiled with the -mwindows option, so that it
is a GUI application, not a console application, and doesn't show a
console window in any case. This workaround doesn't work for pluggable
transports, because they need to be able to write control messages to
stdout.

https://trac.torproject.org/projects/tor/ticket/9444#comment:30