Implement a function openconnect_set_useragent

Hello,

I have a request to propose: to help the software that use openconnect as backend to launch VPN to tune their Useragent (like the option --useragent), did you think it's possible to create and expose a function openconnect_set_useragent?

For information, I have already perform a branch that implement this new function by I'm not totally sure I can provide a merge request directly.

My branch with the function is here: https://gitlab.com/earendilfr/openconnect/-/tree/add-function-tune-useragent

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