clients: add a helper function to get required openconnect VPN secrets

clients: add a helper function to get required openconnect VPN secrets

OpenConnect needs three secrets - COOKIE, HOST and FINGERPRINT. They can be
obtained by authenticating to the server. This can be performed by running
"openconnect --authenticate <host>" and the three values are printed to stdout.

Note that the function may (probably will) interactively ask user for
his credentials.

Alternatively, it would be possible to dlopen() libopenconnect and call its
functions. However, as that would be more complicated and would also require
implementing functionality that openconnect simply does for free for us, it is
not worth it, I think.