Upgrade cloudabi-utils and related packages to the latest version.

Now that we have ARPC, Flower and yaml2argdata packaged, we can conclude
this series of commits by upgrading cloudabi-utils to the latest
upstream version again. This version of cloudabi-utils now has
integrated support for starting sandboxed processes that perform network
communication using Flower.

As cloudabi-utils has partially been rewritten in C++, drop the
dependency on libyaml. We now make use of yaml-cpp. Also apply some
minor cleanups (e.g., use DISTVERISON as opposed to PORTVERSION).

Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D12265