PR14706: Follow forks in stapdyn and enable process.begin

PR14706: Follow forks in stapdyn and enable process.begin

* stapdyn/mutatee.cxx (mutatee::call_utrace_dynprobe): Newly abstracted
  function to call utrace handlers from various contexts.
  (mutatee::begin_callback): New, runs process.begin callbacks.
  (mutatee::exit_callback): Use call_utrace_dynprobe.
  (mutatee::thread_callback): Use call_utrace_dynprobe.
  (mutatee::copy_forked_instrumentation): New, set all the member
  variables for a newly-forked process.
* stapdyn/mutator.cxx (mutator::run): Trigger process.begin in target.
  (mutator::post_fork_callback): Create an attached mutatee for the
  fork, copy the instrumentation status, and trigger process.begin.