libfdisk: modernize ZFS GPT type description

libfdisk: modernize ZFS GPT type description

The GPT type GUID 6A898CC3-1DD2-11B2-99A6-080020736631 is currently
described as "Solaris /usr & Apple ZFS". This reflects early Solaris
and Apple experiments with ZFS, but today the same GUID is widely used
by OpenZFS implementations on Linux, illumos, FreeBSD, and other
platforms to mark ZFS pool member partitions.

Apple's ZFS work was discontinued long ago, while OpenZFS has become
the actively maintained and de facto standard implementation. Update
the human-readable description to the simpler and more accurate:

    "ZFS pool member"

A short comment is added to note the GUID’s historical Solaris /usr
origin and its brief use by Apple. Only the description string is
changed; the GUID itself and its semantics remain unchanged.