This commit rewrites a bunch of the code to always use *Machine instead
of a mix of both, and a mix of tailcfg.Node and Machine.
Now we use *Machine, and if tailcfg.Node is needed, it is converted just
before needed.
tests for that feature.
Other fixes: clean up a few typos in comments. Fix a bug that caused the
tests to run four times each. Be more consistent in the use of log
rather than fmt to print errors and notices.