flua: add ucl library

libucl comes with a Lua library binding. Build it into flua.

This lets us parse/generate config files in the various formats supported by
libucl with flua. For example, the following script will detect the format of
an object written to stdin as one of UCL config, JSON, or YAML and write it to
stdout as pretty-printed JSON:

local ucl = require('ucl')
local parser = ucl.parser()
parser:parse_string(io.read('*a'))
local obj = parser:get_object()
print(ucl.to_format(obj, 'json'))

Reviewed by: kevans, pstef
Approved by: mmacy (mentor)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D25009