Add support for Google Authenticator 2fa on Juniper VPN

Add support for Google Authenticator 2fa on Juniper VPN

We resently changed our Juniper VPN from SMS 2fa to use Google
Authenticator instead. Before it worked perfectly with "openconnect
--juniper" switch, but after the change all we got was:

Unknown form ID 'frmTotpToken'
and a dump of the form.

I spent some time debugging the issue, and managed to write a very
simple fix for it.

Signed-off-by: Janne Juntunen <janne.juntunen@hermanit.fi>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>