Enhance QSsl::Protocol enumeration to enable selection of SslV3 and TlsV1 - leaving SslV2 out

With the insecurities in SslV2 it would be handy to be able to select to not use it but instead use SslV3 or TlsV1.

What I propose is adding a NoSslV2 element to the QSsl::Protocol enumeration so someone can set the protocol on a QSslSocket to use the secure protocols.
This is possible using the openssl option for setting the SSL_OP_NO_SSLv2.

I have attached a patch that appears to do the job in the testing that I have done.
The patch touches
src/network/ssl/qssl.h to add the enum element
src/network/ssl/qsslsocket_openssl.cpp to set the ssl CTX option if the NoSslV2 is desired.