Bring in a simple event tracing library and POC utility

Bring in a simple event tracing library and POC utility

- Import libevtr, a library for abstracting access to an event stream.
  libevtr uses its own dump format and can synthesize event attributes
  based on known event types.
- Modify ktrdump(8) to be able to dump an event stream to a file
  using libevtr.
- Add evtranalyze(1), a proof of concept utility to display events in
  a line-oriented text format or to generate an svg file displaying
  the events on each processor. This needs quite some work.