Allowing unsafe rehandshakes used to be required for web compatibility,
but this is no longer a concern in 2018. So there should no longer be
compatibility benefits to calling this function. All it does is make
your TLS connection insecure.

Also, rehandshaking no longer exists at all in TLS 1.3.

At some point (maybe soon!) glib-networking will begin ignoring the
rehandshake mode, so let's deprecate it now.