Add an additional, libucl-based configuration file parser to ctld.

Default ctld behavior remains unchanged - libucl parser can be selected
explicitly by adding -u switch to ctld command line.

Reviewed by: trasz
Approved by: trasz (mentor)
MFC after: 1 month
Relnotes: yes
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D4534