The ClearCase SCM backend invokes cleartool a lot, and every time was

assembling a big Popen call and duplicating error handling. This change

centralizes all of that into a single method, while also improving

unicode/bytes compatibility, logging, and console shell handling for

windows.
The windows issue is the most interesting one here. Some windows

applications are marked as requiring a console (as opposed to using

windows), and invoking them would always pop up a console window,

regardless of whether input and output was actually possible.

cleartool is one such executable. An existing workaround had been

added to use shell=True, but this was only enabled when using Python

2.7 and Windows 7 (the initial patch was not written with

future-proofing in mind), and relying on the shell opens up a variety of

other potential issues, including security concerns. The startupinfo

flag to Popen allows actually controlling this at a lower level.

Ran unit tests.
Tested that startupinfo changes to Popen invocation did the correct

  thing for Windows hosts.