vtls: reinstantiate engine on duplicated handles

Handles created with curl_easy_duphandle do not use the SSL engine set
up in the original handle. This fixes the issue by storing the engine
name in the internal url state and setting the engine from its name
inside curl_easy_duphandle.

Reported-by: Anton Gerasimov
Signed-of-by: Laurent Bonnans
Fixes #2829
Closes #2833