SSL/TLS: Add support for enforcing a TLS version through a new sslVersion field in the MQTTClient_SSLOptions structure. This affects which *_client_method() function is called in SSLSocket.c: 0 (default) -> SSLv23_client_method(), 1 -> TLSv1_client_method(), 2 -> TLSv1_1_client_method(), 3 -> TLSv1_2_client_method()
Signed-off-by: Günter Obiltschnig <guenter.obiltschnig@appinf.com>