Add Argdata.

The existing cloudabi-utils package has a copy of a library called
Argdata integrated. Future versions of cloudabi-utils will no longer
ship with it, as Argdata can be used independently of CloudABI.

This change introduces a new devel/argdata port, which can be used by
future versions of cloudabi-utils.

Approved by: bapt
Differential Revision: https://reviews.freebsd.org/D9799