Add zfsboot to the scripted interface of bsdinstall(8); oops!

NB: If the zfsboot variables ($ZFSBOOT_*) are set, a script is
assumed to want zfsboot module instead of scriptedpart module.

Submitted by:	Loïc Brarda <loic.brarda@cern.ch>
Reviewed by:	nwhitehorn@
MFC after:	3 days

usr.sbin/bsdinstall/scripts/script

@@ -95,7 +95,11 @@ fi
 # Make partitions
 rm -f $PATH_FSTAB
 touch $PATH_FSTAB
-bsdinstall scriptedpart "$PARTITIONS"
+if [ "$ZFSBOOT_DISKS" ]; then
+	bsdinstall zfsboot
+else
+	bsdinstall scriptedpart "$PARTITIONS"
+fi
 bsdinstall mount
 
 # Unpack distributions