Remove support for using wait3() in place of waitpid().

All Unix-oid platforms that we currently support should have waitpid(),
since it's in V2 of the Single Unix Spec.  Our git history shows that
the wait3 code was added to support NextStep, which we officially dropped
support for as of 9.2.  So get rid of the configure test, and simplify the
macro spaghetti in reaper().  Per suggestion from Fujii Masao.