Updated the TLS negotiation logic to adaptively try to connect using
the highest TLS version supported by both client and server.
Previously, OpenVPN would always connect using TLS 1.0.

Also added tls-version-min directive to force a higher TLS version
than 1.0:

tls-version-min <version> ['or-highest'] -- sets the minimum
TLS version we will accept from the peer.  Examples for version
include "1.0" (default), "1.1", or "1.2".  If 'or-highest' is
specified and version is not recognized, we will only accept
the highest TLS version supported by the local SSL implementation.

Examples:

tls-version-min 1.1 -- fail the connection unless peer can
  connect at TLS 1.1 or higher.

tls-version-min 1.2 or-highest -- require that the peer
  connect at TLS 1.2 or higher, however if the local SSL
  implementation doesn't support TLS 1.2 (as it wouldn't
  if linked with an older version of OpenSSL), reduce the
  minimum required version to the highest version supported
  by the local SSL implementation (such as TLS 1.0).  This
  is intended to allow client configurations to target higher
  TLS versions that are supported on the server, even if some
  older clients don't support these versions yet.
Added "setenv opt" directive prefix.  If present, and if the
directive that follows is recognized, it will be processed
as if the "setenv opt" prefix was absent.  If present and if
the directive that follows is not recognized, the directive
will be ignored rather than cause a fatal error.

For example, suppose you are distributing a client
configuration file and want to set the minimum TLS version
that the client requires from the server to 1.2.

By using the following directive,

  setenv opt tls-version-min 1.2 or-highest

only newer clients that understand the tls-version-min directive
would process it, while older clients would ignore it.