Replace ZFS on Linux references with OpenZFS

This change updates the documentation to refer to the project
as OpenZFS instead ZFS on Linux.  Web links have been updated
to refer to https://github.com/openzfs/zfs.  The extraneous
zfsonlinux.org web links in the ZED and SPL sources have been
dropped.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11007

cmd/arc_summary/arc_summary2

@@ -42,7 +42,7 @@
 Provides basic information on the ARC, its efficiency, the L2ARC (if present),
 the Data Management Unit (DMU), Virtual Devices (VDEVs), and tunables. See the
 in-source documentation and code at
-https://github.com/zfsonlinux/zfs/blob/master/module/zfs/arc.c for details.
+https://github.com/openzfs/zfs/blob/master/module/zfs/arc.c for details.
 """
 
 import getopt