rc: Add a zpoolreguid rc.d script

If one boots up multiple copies of a template VM image containing a
zpool, the pool GUIDs will be identical, making it impossible to, e.g.,
share datasets between them.

This diff introduces a simple workaround for the problem: one can use
the script to, upon first boot, assign a new GUID to one or more zpools.
This will be useful when building ZFS-based VM images from release(7).

Reviewed by:	mav, allanjude, asomers
Reviewed by:	Pau Amma (docs)
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35336

libexec/rc/rc.conf

@@ -69,6 +69,8 @@ rc_conf_files="/etc/rc.conf /etc/rc.conf.local"
 zfs_enable="NO"		# Set to YES to automatically mount ZFS file systems
 zfskeys_enable="NO"	# Set YES to autoload ZFS encryption keys
 zfs_bootonce_activate="NO" # Set YES to make successful bootonce BE permanent
+zpool_reguid=""		# Set to zpools for which the GUID should be replaced
+			# upon first boot.
 
 # ZFSD support
 zfsd_enable="NO"	# Set to YES to automatically start the ZFS fault

libexec/rc/rc.d/Makefile

@@ -326,6 +326,7 @@ ZFS+=		zfsbe
 ZFS+=		zfsd
 ZFS+=		zfskeys
 ZFS+=		zpool
+ZFS+=		zpoolreguid
 ZFS+=		zvol
 ZFSPACKAGE=	rc
 .endif

libexec/rc/rc.d/zpoolreguid

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+# PROVIDE: zpoolreguid
+# REQUIRE: zpool
+# BEFORE: mountcritlocal
+# KEYWORD: firstboot nojail
+
+. /etc/rc.subr
+
+name="zpoolreguid"
+desc="Generate a new zpool GUID"
+rcvar="zfs_enable"
+start_cmd="zpoolreguid_start"
+
+zpoolreguid_start()
+{
+	local pool
+
+	for pool in "${zpool_reguid}"; do
+		zpool reguid $pool
+	done
+}
+
+load_rc_config $name
+run_rc_command "$1"