Add coverage for spawning GUI subsystem programs

[rprichard]

The current docs leave this open:

I have not tested whether or how these flags affect non-console programs (i.e. programs whose PE header subsystem is WINDOWS rather than CONSOLE).

I did some exploration of this issue while digging into a winpty bug involving ConEmu.

I did very minimal testing, and only on WIndows 7 SP1 64-bit. The behavior I saw when spawning a process with a GUI executable was:

• Specifying both CREATE_NEW_CONSOLE and DETACHED_PROCESS is still an error.
• The flags are otherwise ignored, and CreationConsoleMode is Detach.
• I can still use STARTF_USESTDHANDLES to set an stdout handle.