Some minor changes to the "tunala" demo.

* Seal off some buffer functions so that only the higher-level IO functions
  are exposed.

* Using the above change to buffer, add support to tunala for displaying
  traffic totals when a tunnel closes. Useful in debugging and analysis -
  you get to see the total encrypted traffic versus the total tunneled
  traffic. This shows not only how much expansion your data suffers from
  SSL (a lot if you send/receive a few bytes at a time), but also the
  overhead of SSL handshaking relative to the payload sent through the
  tunnel. This is controlled by the "-out_totals" switch to tunala.

* Fix and tweak some bits in the README.

Eg. sample output of "-out_totals" from a tunnel client when tunneling a brief
"telnet" session.

Tunnel closing, traffic stats follow
    SSL (network) traffic to/from server;     7305 bytes in,     3475 bytes out
    tunnelled data to/from server;            4295 bytes in,      186 bytes out