Fix trusted x11 forwarding with client xauth data

[Joerger]

Previously, tsh ssh -Y assumed that there was no required xauth data set for the current client $DISPLAY. This is usually the case for local XServers, which will default to connecting with local authentication mechanisms. However, in each x11 forwarding server jump, we set some fake xauth data for client-server authentication. This fake xauth data must be propogated to the next server jump, or else the client will fail to connect to the XServer proxy, leading to this error: X11 connection rejected because of wrong authentication..

This PR updates tsh to use existing xauth data when found, instead of always defaulting to creating new fake xauth data. Note that this now properly matches the openSSH implementation.

Fixes #48920

Changelog: Fix tsh ssh -Y when jumping between multiple servers.

[aws-amplify-us-west-2]

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