Similar to libexec/, do the same with lib/. Make WARNS=6 the norm and
lower it when needed.
I'm setting WARNS?=0 for secure/. It seems secure/ includes the
Makefile.inc provided by lib/. I'm not going to touch that directory.
Most of the code there is contributed anyway.
Beyond changes to the build system, this includes fixing up the sample
freebsd.mc configuration for changes in defaults and syntax, removing
outdated documentation, and updating the release notes.