flua/makeman reviews

Hi,

I have a couple of reviews in flight for flua && makeman, if anybody
is interested:

- https://reviews.freebsd.org/D39083 (flua: lposix: add more useful
functions for general purpose scripts)
- https://reviews.freebsd.org/D39084 (tools: build: add a rewrite of
makeman in lua)

D39084 doesn't hook up makeman.lua to the "makeman" target, yet; it
simply adds the new version, then we can validate it and replace the
existing makeman shell script.

The main benefit of the rewrite of makeman is that it applies some
super-naive parallelism to makeman; one fork for every option
described in src.conf(5). This dropped execution time on one of our
high-core count arm64 machines from ~1m40s on average to ~15s. In a VM
on an Intel NUC, down from ~2 minutes to 40s.

Thanks,

Kyle Evans