1113598: Correct date ranges when global attrs are in play

We were checking if single entitlements or stacks were valid, which doesn't take global attributes (like guest limit) from unrelated entitlements into account.

You can reproduce this by attaching  a guest limit subscription while you're over the limit of allowed guests, then attaching a hypervisor sub (anything with -1 guest limit, or more than you've got active).  We fail to properly compute valid date ranges.
:ok: seems to make the dates inclusive. Seems fair.