Add support to change verify mode on SSL instance

Motivation:

To support SSLEngine.set*AuthMode(...) we need to add support to change the verify mode of an SSL instance.

Modifications:

Add SSL.setVerify(...) which allows to change the verify mode of an already created SSL instance.

Result:

It's possible now to change the verify mode on an created SSL instance and so a more complete SSLEngine implementation can be done.
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