Fix DhParams copy-assignment and copy-constructor

DhParams copy-assignement was implemented as non-op operator.
This is weird for a copy to do nothing!
No intends are indicated to help.
Seems only done to help MSVC compiler, but this is WRONG
and cause a SECURITY FAILURE as the DhParam is not set
as supposed to be.
Consequence observed on UWP daemon: the anonymous connection
cannot be done and the certificate is always send in cleartext.

The fix consists in using the dedicated gnutls API
to copy correctly the internal dh_params C structure.
This is used to implement copy-assignement and copy-constructor
methods

Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>
Change-Id: I49d3a3fd2c0a2ed5fde0bd251efbad589c830c70