libutil++: Move to clibs

This library only installs manual pages, so putting it in its own
package means we build a FreeBSD-libutil++-man package but not a
base FreeBSD-libutil++ package. Without a base package, the man
package can't be installed due to the missing dependency.

We don't really need a separate package for a few manpages, so move
it to clibs.

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D51756