Show instance for System.Process.CreateProcess and CmdSpec

Looking at the source there doesn't seem to be any particular reason to omit Show. My best guess is that it's since some details are platform dependent, but since Show is basically for debugging, it doesn't seem like a blocker.

Also it would be nice to be able to get a PID from a ProcessHandle. I notice this one is more heavily system specific and it has an awkward mvar but it would be nice to get some kind of indication about which process just died other than the name I started it with.

If there's no fundamental objection I can try a patch.

Start by reading the definitions of System.Process.CreateProcess, CmdSpec, and ProcessHandle in the source, then trace the platform-specific process-handling code. Confirm how Show output can remain useful across platforms and how a process identifier could be exposed; done means the requested information is available without relying only on the started process name.

Written by the indexing model from the issue text.