Merge from upstream at 4189ef5d from https://github.com/onetrueawk/awk.git

Note: this backs out a number of changes we've made to awk because
they aren't upstream, but are on the vendor branch. Those will be
reapplied. svn makes it needlessly difficult to know which ones, but
at least r315426, r301289, and maybe r301691, though there may be
others too. None of these are critical, so bisecting through this
point is safe for all but awk regression tests :).