Just attaching the current patch.  Thanks for the input Dmitry and
Kaushal.  I'll leave it a while to see whether anyone else wishes to
weigh in on the outstanding items:

* The inconsistent predicate naming of `trace-is-traced' vs
  `trace-is-traceable-p'.
  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27397#19

* Whether foreground-tracing variants of the new commands are wanted.
  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27397#25
  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27397#34

* Whether it's a good idea to display the trace buffer initially
  when one of the background tracing commands is invoked.


(I guess I'll follow up next weekend if no one has anything to add
in the interim.)


-Phil