Import latest one-true-awk from upstream

Import git hash 4189ef5d from https://github.com/onetrueawk/awk.git as
there's not been a release in a while.

Upstream one-true-awk woke-up! Time to catch up. This may also revert
FreeBSD changes that we'd placed in the vendor branch in anticipation
of their inclusion in upstream. That's not yet the case, and these
will be resolved in the merge.

See FIXES for a complete list of bugs fixed (starting with the Jun 7,
2018 entry).