Rename GTFO to GOAWAY

This is almost a straight revert of the various GTFO -> GOAWAY changes. It seems like this is the popular move as per the discussion on the httpbis mailing list.

Arguments for the revert:

1. GOAWAY is self-descriptive and serves it purpose well.
2. Editorial changes that don't clearly improve the spec should be discouraged so we can reach a stable spec sooner (granted this change doesn't affect the wire at all, but documentation churn counts too).
3. GTFO is crass and doesn't belong in internet standards.
4. GTFO, if it really stands for "General Termination of Future Operations," has a surprising definition and may produce confusion. We should strive for absolute clarity above all else in editorial changes.