Use p11-kit for directing PIN request to process_auth_form()

Use p11-kit for directing PIN request to process_auth_form()

Set a 'pin-source' attribute which identifies the vpninfo structure, and
register a handler which converts it to an auth form for the GUI to process.

If the URI we are given already contains a pin_source then theoretically
we don't override it; we assume the caller knew what they were doing. In
practice, p11_kit_get_pin_source() seems to be returning NULL even when
the attribute *is* set, so we always override it.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>