Restore ability to renegotiate

[ocheron]

This is a fix for #161.

The handshake behaviour is slightly modified because the implementation will now accept to start a second handshake replacing a first handshake that is not finished yet. I don’t think this can cause any harm, especially with secure renegotiations. But if required it should also be possible to use more complex logic or add new state to discriminate between finished and on-going handshake cases.

[ocheron]

The test suite contains renegotiation tests but does not demonstrate the bug because tls renegotiates with itself. I tested manually against OpenSSL like this:

Server-initiated renegotiation

• [Terminal 1] Start OpenSSL server:

  $ openssl req -x509 -subj /CN=localhost -new -nodes -out cert.pem
  $ openssl s_server -key privkey.pem -cert cert.pem

• [Terminal 2] Start tls-simpleclient:

  $ tls-simpleclient --http1.1 --no-validation -t 60000 localhost 4433 -d

• [Terminal 1] Press r + <Return> to trigger renegotiation in s_server

Client-initiated renegotiation

• [Terminal 1] Start tls-simpleserver with new option --allow-renegotiation (added in commit ocheron/hs-tls@76f87cc):

  $ tls-simpleserver --key privkey.pem --certificate cert.pem -t 60000 \
    --allow-renegotiation 4433

• [Terminal 2] Start OpenSSL client:

  $ openssl s_client

• [Terminal 2] Press R + <Return> to trigger renegotiation in s_client

[kazu-yamamoto]

Sorry for the delay.
Looks great to me!

[kazu-yamamoto]

@ocheron Do you want to merge this PR by yourself?

[ocheron]

OK I'll merge it, but first I'll also take some time to remember what was the possible improvement and create an issue to track it.