Permit customization of secure transport layer in DcmTLSSCU

[jogerh]

Today, DcmTLSSCU requires certificate and key files to enable TLS.
The Windows platform has its own certificate store, and exporting all
certificates to the filesystem may not be desireable. Instead, we want
to populate the OpenSSL certificate store in memory from the Windows
certificate store.

This change allow us to derive from DcmTLSSCU to customize how TLS
transport layers are created and configured before we leave it over
to the default DcmTLSSCU::initNetwork implementation.

[jogerh]

Closing because a better option is to not use DcmTLSSCU. We achieve the same in a more elegant way by inheriting from DcmSCU, and provide the SSL context through DcmSCU::useSecureConnection