Add zfs_prepare_disk script for disk firmware install

Have libzfs call a special `zfs_prepare_disk` script before a disk is
included into the pool.  The user can edit this script to add things
like a disk firmware update or a disk health check.  Use of the script
is totally optional. See the zfs_prepare_disk manpage for full details.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15243

scripts/zfs_prepare_disk

@@ -0,0 +1,17 @@
+#!/bin/sh
+#
+# This is an optional helper script that is automatically called by libzfs
+# before a disk is about to be added into the pool.  It can be modified by
+# the user to run whatever commands are necessary to prepare a disk for
+# inclusion into the pool.  For example, users can add lines to this
+# script to do things like update the drive's firmware or check the drive's
+# health.  The script is optional and can be removed if it is not needed.
+#
+# See the zfs_prepare_disk(8) man page for details.
+#
+# Example:
+#
+# echo "Prepare disk $VDEV_PATH ($VDEV_UPATH) for $VDEV_PREPARE in $POOL_NAME"
+#
+
+exit 0