[PATCH] Support for TLSv1.1 and 1.2

This patch seeks to add support for TLSv1.1 and TLSv1.2 options and
methods alongside TLSv1.0 when OpenSSL v1.0.1+ is being built against.

It also updates the documentation for sslproxy_version which was not
mentioning what the supported version codes were.

Future work:
  * make version config option(s) accept a set of named versions and
convert to codes internally.
  * redesign how version and options are handled. Admin should be able
to just list the TLSv* wanted and Squid figure out the appropriate
options from there.

Amos