Detect and deny excess renegotiations attempts.

Detect and deny excess renegotiations attempts.

Switch 'server_handshake_count' from a uint8_t to 2 unsigned int bits.
Since we won't ever be doing more than 3 handshakes, we don't need the
extra space.

Toggle tor_tls_t.got_renegotiate based on the server_handshake_count.
Also assert that when we've done two handshakes as a server (the initial
SSL handshake, and the renegotiation handshake) we've just
renegotiated.

Finally, in tor_tls_read() return an error if we see more than 2
handshakes.