Add zgenhostid utility script

Turning the multihost property on requires that a hostid be set to allow
ZFS to determine when a foreign system is attemping to import a pool.
The error message instructing the user to set a hostid refers to
genhostid(1).

Genhostid(1) is not available on SUSE Linux.  This commit adds a script
modeled after genhostid(1) for those users.

Zgenhostid checks for an /etc/hostid file; if it does not exist, it
creates one and stores a value.  If the user has provided a hostid as an
argument, that value is used.  Otherwise, a random hostid is generated
and stored.

This differs from the CENTOS 6/7 versions of genhostid, which overwrite
the /etc/hostid file even though their manpages state otherwise.

A man page for zgenhostid is added. The one for genhostid is in (1), but
I put zgenhostid in (8) because I believe it's more appropriate.

The mmp tests are modified to use zgenhostid to set the hostid instead
of using the spl_hostid module parameter.  zgenhostid will not replace
an existing /etc/hostid file, so new mmp_clear_hostid calls are
required.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #6358
Closes #6379

man/man8/Makefile.am

@@ -4,6 +4,7 @@ dist_man_MANS = \
 	vdev_id.8 \
 	zdb.8 \
 	zfs.8 \
+	zgenhostid.8
 	zinject.8 \
 	zpool.8 \
 	zstreamdump.8

man/man8/zgenhostid.8

@@ -0,0 +1,72 @@
+.\"
+.\" CDDL HEADER START
+.\"
+.\" The contents of this file are subject to the terms of the
+.\" Common Development and Distribution License (the "License").
+.\" You may not use this file except in compliance with the License.
+.\"
+.\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+.\" or http://www.opensolaris.org/os/licensing.
+.\" See the License for the specific language governing permissions
+.\" and limitations under the License.
+.\"
+.\" When distributing Covered Code, include this CDDL HEADER in each
+.\" file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+.\" If applicable, add the following below this CDDL HEADER, with the
+.\" fields enclosed by brackets "[]" replaced with your own identifying
+.\" information: Portions Copyright [yyyy] [name of copyright owner]
+.\"
+.\" CDDL HEADER END
+.\"
+.\"
+.\" Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+.\"
+.Dd July 24, 2017
+.Dt ZGENHOSTID 8 SMM
+.Os Linux
+.Sh NAME
+.Nm zgenhostid
+.Nd generate and store a hostid in
+.Em /etc/hostid
+.Sh SYNOPSIS
+.Nm
+.Op Ar hostid
+.Sh DESCRIPTION
+If
+.Em /etc/hostid
+does not exist, create it and store a hostid in it.  If the user provides
+.Op Ar hostid
+on the command line, store that value.  Otherwise, randomly generate a
+value to store.
+.Pp
+This emulates the
+.Xr genhostid 1
+utility and is provided for use on systems which do not include the utility.
+.Pp
+.Sh OPTIONS
+.Op Ar hostid
+Specifies the value to be placed in
+.Em /etc/hostid .
+It must be a number with a value between 1 and 2^32-1.  This value
+.Sy must
+be unique among your systems.  It must be expressed in hexadecimal and be
+exactly 8 digits long.
+.Sh EXAMPLES
+.Bl -tag -width Ds
+.It Generate a random hostid and store it
+.Bd -literal
+# zgenhostid
+.Ed
+.It Record the libc-generated hostid in Em /etc/hostid
+.Bd -literal
+# zgenhostid $(hostid)
+.Ed
+.It Record a custom hostid (0xdeadbeef) in Em etc/hostid
+.Bd -literal
+# zgenhostid deadbeef
+.Ed
+.El
+.Sh SEE ALSO
+.Xr spl-module-parameters 5 ,
+.Xr genhostid 1 ,
+.Xr hostid 1

man/man8/zpool.8

@@ -732,7 +732,7 @@ in the
 man page.  In order to enable this property each host must set a unique hostid.
 See
 .Xr genhostid 1
-and
+.Xr zgenhostid 8
 .Xr spl-module-paramters 5
 for additional details.  The default value is
 .Sy off .