To fix a major security problem related to incorrect use of
SSL/TLS renegotiation, OpenSSL has turned off renegotiation by
default.  We are not affected by this security problem, however,
since we do renegotiation right.  (Specifically, we never treat a
renegotiated credential as authenticating previous communication.)
Nevertheless, OpenSSL's new behavior requires us to explicitly
turn renegotiation back on in order to get our protocol working
again.

Amusingly, this is not so simple as "set the flag when you create
the SSL object" , since calling connect or accept seems to clear
the flags.

For belt-and-suspenders purposes, we clear the flag once the Tor
handshake is done.  There's no way to exploit a second handshake
either, but we might as well not allow it.