This change adds a new exec package thats main goal is to run
external processes on Windows. Unfortunately due to a couple
things that can't be accomplished with the stdlib os/exec package,
this new package is meant to replace how processes for host process
containers are launched.

The main shortcomings are not being able to pass in a pseudo console to use
for tty scenarios, and not being able to start a process assigned to a job object
instead of doing the Create -> Assign dance. Both of these issue are centered
around not having access to the process thread attribute list that is setup inside
of syscall.StartProcess. This is needed to be able to properly setup both cases,
as it requires calling UpdateProcThreadAttribute and passing in what's necessary
for both scenarios.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

* Remove conpty comments
* Terminate the process if os.FindProcess doesn't work.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

This change adds pseudo console support to the exec package.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

x/sys/windows has the Proc thread attribute work we need already, so swap
to this instead of our own definitions.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

1. Add comment regarding thread safety
2. Add check on job object test to make sure the pids slice has at least
one element before testing what's in [0].
3. Change stdioOurEnd -> stdioPipesOurSide and
stdioProcEnd -> stdioPipesProcSide.
4. Misc. clarification comments on fields and methods.
5. Change from raw string literal in powershell test to string with carriage
return at the end.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

1. Add nod to using e.ExitCode() in the process lifecycle comment
2. Change to different way to defer close the thread handle to avoid
nolint comment

Signed-off-by: Daniel Canter <dcanter@microsoft.com>