Include files for pkgbase

I was looking int
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254173 recently
and realised that packaging for include files is messed up. For instance,
<stdio.h> lives in runtime-dev but it includes <xlocale/_stdio.h> which is
in utilities-dev. The clibs-dev package containers C++ header files, most
of which cannot work without the standard C includes from runtime-dev.
Looking closer, include files from src/include are in runtime but
subdirectories of src/include don't have PACKAGE set, to drop into
utilities by default.

This is all too confusing. I think we should put all the includes from
src/include and subdirectories as well as the C++ standard includes into
the same package and I think that package should be clibs-dev since none of
it is much use without the libs contained in clibs-dev.