[X2Go-Dev] Support SSH_AUTH_PARTIAL

I'm starting to take a stab at supporting SSH_AUTH_PARTIAL in x2goclient. Here's what I have at the moment. I think there still is work to be done for userAuthKeyboardInteractive() and userChallengeAuth() and userAuthWithPass(), but they would be similar to what was done for userAuthAuto and userAuthWithKey.

This also requires work in libssh - see https://bugs.libssh.org/T82

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