VOMS AC lifetime check

[sustr4]

Taken over from GGUS: https://ggus.eu/index.php?mode=ticket_info&ticket_id=120618 by @maarten-litmaath

please have a look at this other ticket about problematic proxies:

https://ggus.eu/index.php?mode=ticket_info&ticket_id=120463

We suspect libgridsite (or rather libcanl?) insists that the VOMS AC
shall have a lifetime that does not exceed the lifetime of its VOMS
server host certificate: can you check what the code does?

Other libraries (VOMS, Argus) appear to be satisfied if the VOMS AC
and the VOMS server host certificate are still valid when the check
is being done, and that latter approach would be preferred from an
operations point of view.