bin/119976: [patch] Add ZFS support to gpt(8)

[Niki Denev]

http://www.freebsd.org/cgi/query-pr.cgi?pr=bin/119976

I'm running here with similar patch and it works ok.

But the changes to migrate.c in the PR's patch will probably won't do anything
because FS_ZFS is not defined in /usr/include/sys/disklabel.h

There was some discussion about adding it :
http://readlist.com/lists/freebsd.org/freebsd-current/8/44907.html