Add openconnect_get_connect_url(), use it in --authenticate output

Add openconnect_get_connect_url(), use it in --authenticate output

As noted in the long comment in openconnect.h, we need to provide the
actual *URL* for the connection including the real hostname. And that
means we also need to provide a suitable --resolve argument to pass on
to the connecting openconnect.

Add openconnect_get_connect_url() for the GUI auth-dialogs to use, and
make 'openconnect --authenticate' emit the same. If we just put it into
$HOST that might break existing setups that don't use the newly-added
$RESOLVE variable too, so put it in another new variable $CONNECT_URL
and leave the original $HOST alone.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>