Send SIGINT on KeyboardInterrupt

envdir should receive the SIGINT, instead of sending SIGTERM to the
process.
Wait for the process to finish, before sending SIGKILL

The subprocess might need some time to exit (e.g. supervisor), and
envdir should not send SIGKILL just after SIGTERM/SIGINT.
Keep file descriptors with Popen (for `is_windows`)

Closing file descriptors makes envdir impossible to use with chaussette,
and I cannot see why that should get done - only that it has been
changed to `not is_windows` for some reason.

Fixes #16
Merge branches 'do-not-close-fds' and 'fix-kill'