Get rid of cur and pending pointers in struct sudo_event_base. We

Get rid of cur and pending pointers in struct sudo_event_base.  We
now pop the first event off the active queue instead of using a
foreach loop with deferred removal of the event.
Add SUDO_EVQ_INSERTED and SUDO_EVQ_TIMEOUTS flags to indicate that
the event on the event queue and timeouts queue respectively.
No longer need to compare the timeout to {0,0} or compare the
event's base pointer to NULL to determine queue membership.