There was a potential race where two handlers could be added at
the same time. Go Maps are not thread-safe, so that could do
unpleasant things. Add a mutex to keep things safe.
Also, swap the order or Register and Start for the handlers in
Libpod runtime created. As written, there was a small gap between
Start and Register where SIGTERM/SIGINT would be completely
ignored, instead of stopping Podman. Swapping the two closes this
gap.
Signed-off-by: Matthew Heon <mheon@redhat.com>