Cleanup event queue, don't expose nagios_squeue

nagios_squeue is a variable local to the event queue, and should not be
exposed outside the event queue.

That means, the API for the event queue needs to be cleaned up so it
doesn't require a squeue as an argument, which makes sense for a
singleton event queue.

Signed-off-by: Max Sikstrom <msikstrom@op5.com>