Rdp: add an option to set TLS security level
This is equivalent to pass the /tls:seclevel parameter to freerdp. This parameter accepts a value from 0 to 5 that freerdp then passes to this openssl function: https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_security_level.html Setting this value to 0 is needed to connect to some rdp servers, eg. on old versions of MS Windows. I tested the change on a Windows 2008 server.