New port: devel/yaml2argdata.

The latest release of cloudabi-utils has cloudabi-run rewritten from C
into C++. It now also has integrated support for attaching processes to
CloudABI's networking daemon, Flower.

In the process, code specific to parsing YAML configuration files
describing process configuration and resource dependencies has been
placed into a separate library, called yaml2argdata. This has the
advantage that this code can now be used by CloudABI's Kubernetes daemon
as well. Add a package for it, so sysutils/cloudabi-utils can make use
of it when we bump it.

Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D12164