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This does not change any of the logic to apply encryption yet, ie. that
is still tied to LoginJob. This however now enables moving that logic
step by step to Session and thus enable encryption independent of a login
being required.

This was previously done in different places, or worse, in two with
conflicting values.

We can enter startSsl() multiple times, when a socket is reconnected in
Session, which then would result in multiple connections. Connecting it
earlier OTOH shouldn't hurt, so connect it together with the rest of the
socket signals after construction.

We'll also need this for starting encryption earlier than in LoginJob.

This is a leftover from when this was using KTcpSocket, and nowadays was
merely passing around what is the default anyway.

It's only needed for unit tests, and makes reasoning about the session
state code harder.

GIT_SILENT