When annex.stalldetection is set, and git-annex transferrer is used,
a ctrl-c does not propagate to the transferrer process.

The result is that, the next time the process sends a message to its output
handle (eg a progress update), it gets a SIGINT, and so an ugly message is
output to the console, after the user was returned to the prompt.

The SIGINT is not propagated because a child process group is used for
git-annex transferrer, in order to let child processes of it be killed
along with it when a stall is detected.

Maybe what's needed is a SIGINT handler in the main git-annex that
signals all the transferrer processes with SIGINT and waits on them
exiting. And other signals, eg SIGTSTP for ctrl-z.

> Implemented this, but not for windows (yet). But not gonna leave open
> for something that on windows in my experience does not work very
> reliably in general. (I've many times hit ctrl-c in a windows terminal and
> had the whole terminal lock up.) So, [[done]] --[[Joey]]

Or, note that it would suffice to remove the child process group stuff,
if we assume that all child processes started by git-annex transferrer are
talking to a pipe, and will output something, eg a progress update,
and so receive a SIGPIPE once the transferrer process has caught the
SIGINT and exited.
[[todo/stalldetection_does_not_work_for_rsync_and_gcrypt]] would be a
prereq for this approach. But, might there be long-running child processes
that are not on a pipe, and that need to be shutdown on a stall, too?
