auth-dialog: Introduce FORMCHOICE helper macro

libopenconnect >= 3.0 uses a pointer array so we need to change the way
the "choices" field is accessed.
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.]
auth-dialog: Add support for OC_FORM_OPT_IGNORE

Hide any options that libopenconnect doesn't want us to display.