gpart(8): Recognize apple-zfs and solaris-reserved partition ids

Introduce G_PART_ALIAS_SOLARIS_RESERVED, GPT_ENT_TYPE_SOLARIS_RESERVED et al.,
to make gpart show output more convenient on systems with illumos/openindiana
disks visible.

Submitted by: Juraj Lutter <otis AT sk.FreeBSD.org>
Reviewed by: bcr(manpages), delphij, myself
Differential Revision: https://reviews.freebsd.org/D26012