Disable cancellation of sync handshakes
Disable cancellation of sync handshakes

I don't know how to make cancellation work unless we guard access to the
handshake_context using a mutex. The code is too complex already, and I
don't want to add another mutex. I think this will be a lot easier to
fix once we have all TLS operations running on one secondary thread.

Hopefully fixes #97

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking