Fix GlobalProtect authgroup handling

Fix GlobalProtect authgroup handling

When connecting to a GlobalProtect server via the portal interface, then
`vpninfo->authgroup` needs to be set to the URL of one of the allowed
gateways.

The problem here is that if the user actually wanted to select the _first_
gateway in the dropdown list, it was already pre-selected, and thus clicking
"continue"/"login" on the form wouldn't trigger `OC_FORM_RESULT_NEWGROUP`.

This would prevent `vpninfo->authgroup` from getting set correctly, and the
gateway redirect would be skipped entirely.  Thus it was effectively
impossible to select the first option in the gateway dropdown.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>