tcptls: use TLS_client_method with OpenSSL 1.1

OpenSSL 1.1 introduced TLS_client_method() and deprecated the previous
version-specific methods (such as TLSv1_client_method(). Other than
being simpler to use and more correct (gain support for TLS newer that
TLS1, in our case), the older ones produce a deprecation warning that
fails the build in dev-mode.

Change-Id: I257b1c8afd09dcb0d96cda3a41cb9f7a15d0ba07