currently FreeBSD-utilities depends on both FreeBSD-ufs and FreeBSD-zfs.
this is not desirable, because those are both relatively large packages
and the user may want to remove one or the other – or perhaps both, e.g.
in a jail or embedded system.

the reason for this dependency is that fstyp(8), which is in
FreeBSD-utilities, links both libufs and libzfs. FreeBSD-utilities is
the correct place for fstyp, so we don't want to move that.

instead, add two new packages: libufs contains libufs, and libzfs
contains libzfs plus the ZFS libraries it depends on: libavl, libnvpair,
libspl, libtpool, libumem, libuutil, libzfs_core and libzutil.

with this change, it is possible to remove FreeBSD-ufs and/or
FreeBSD-zfs while leaving FreeBSD-libufs, FreeBSD-libzfs and
FreeBSD-utilities installed.