libutil++: Move to the utilities package

We should keep the clibs package as small as possible. Currently it
contains some "extra" libraries, but this isn't a good argument for
adding more stuff. Move it to the utilities package (the default for
bsd.lib.mk consumers), since libutil++ is an INTERNALLIB and thus only
installs man pages, so therefore doesn't need to be in FreeBSD-runtime.

Requested by: kib
Reviewed by: ivy, des
Fixes: f4fd2aa07cde ("libutil++: Move to clibs")
Differential Revision: https://reviews.freebsd.org/D51898