libevent - Move opal_event_include logic to base

[jjhursey]

Per comment on PR #2904 - The logic supporting MCA parameter opal_event_include is the same (nearly) in both the internal and external component. As such it should be moved to base to avoid code duplication.

[jsquyres]

@jjhursey This could probably be classified as a "bug", and therefore be eligible for v2.1.1. Do you want this for v2.1.1, v3.0.0, or beyond? Please update the milestone.

[jsquyres]

(in an effort to clear out the v2.1.0 milestone, I pre-emptively moved this to v2.1.1 -- feel free to update)

[jjhursey]

Let's move it to v3.0 - I don't think it's critical for me to get it into the v2.x series at this point.

[jjhursey]

I doubt I'll get to this in the next week. It would be good to get it into v3.0 though so I'm going to leave the milestone in place for now.

[hppritcha]

Too late for 3.0.0.