sysutils/openzfs*: Switch to merged OpenZFS repo

FreeBSD support has been merged into the openzfs/zfs repo on Github.

Switch the port to use the new upstream. Update maintainer to be me.

Reviewed by: kmoore, mav, wg
Approved by: kmoore (maintainer)
Approved by: mav (mentor)
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D24434