signal handling: refactor some names for clarity

While working on https://github.com/lyft/envoy-mobile/issue/831 @mattklein123 suggested disabling signal handling. I originally thought he meant the signal handling envoy has for fatal signals using `SignalActions`, which didn't make much sense in the context we were working in. While I should have understood he meant signal handling for termination, it didn't help that we have several places to "disable signals", and we do so in bazel config, via compiler directives, and in code. We could clean this up and specify which settings work on fatal signals, which on termination signals, etc.