This makes code using it non-portable, and requires the namespeced
headers from libbsd, instead of any generic system headers. It also
requires more code changes than the overlay mode.
The former used to be the reference implementation, but it has been
stagnant to the point of not showing much signs of life. Switch to
the currently active and more complete implementation for references.
While using fully uppercase man page titles has been the usual
convention for a very long time, it is rather ugly and something that
some other projects are switching away from.
This splits the implementation responsibilities, and reduces embedded
code copies, which was one of the driving points with this project to
start with, so it's nice to give a good example.
We mention that these are now superseded by the glibc implementations,
make the headers cope with already declared functions on glibc-based
systems, and document this in the man pages.
