TS-3195: initial signal handling refactor

  - Move features specific traffic_server from the generic signal
    handling code to traffic_server main.
  - Refactor signal handling into a small API and remove cruft for
    systems that don't support sigaction(2).
  - Move signal handling specific to traffic_server into traffic_server
    main, adopting the small signal handling API.
  - Update other apps that shared traffic_server's signal handling
    policy. In most cases, these don't need to do
    anything special with signal handling.
  - Remove (now) unused proxy.config.stack_dump_enabled.
TS-3195: add crash logging infrastructure and helper

Add the concept of a crash logging helper that is invoked to generate
a detailed crash dump when traffic_server receives a fatal signal.
Add traffic_server infrastructure to start the helper and invoke
it at crash time.

Add traffic_crashlog, a default crash log helper. Document the
whole thing.
TS-3195: CHANGES