bring back default SIGINT handler

This seems to fix a problem I've recently seen where ctrl-c during rsync
leads to `git annex get` moving on to the next thing rather than exiting.

Seems likely that started happening with the switch to System.Process
(d1da9cf2), as the old code took care
to install a default SIGINT handler.

Note that since the bug was only occurring sometimes, I am not 100% sure
I've squashed it, although I seem to have.