auth-dialog: Add support for NEWGROUP
auth-dialog: Add support for NEWGROUP

When the user selects a different authgroup, return NEWGROUP to
libopenconnect to allow it to refresh the form.  Different groups may
require different prompts.

Also, if secondary_username is provided by the server, prepopulate the
form field.

[dwmw2: Update to use form->authgroup_opt and fix crash due to calling
        gtk_dialog_response() from combo_changed callback.]

lkundrak/network-manager-openconnect,lkundrak/network-manager-openconnect