Two minor improvements uncovered by work on upgrading between releases:

* When installing updates, make sure that securelevel <= 0.  Otherwise
  we can't remove the schg flag from files.
* When preparing to download updates, check to see if we already have
  them sitting in the /files/ directory.  This saves bandwidth if users
  run "freebsd-update fetch" more than once without installing updates
  in between.

While I'm here, bump the copyright date.

MFC after:	3 days

usr.sbin/freebsd-update/freebsd-update.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 #-
-# Copyright 2004-2006 Colin Percival
+# Copyright 2004-2007 Colin Percival
 # All rights reserved
 #
 # Redistribution and use in source and binary forms, with or without
@@ -538,6 +538,13 @@ install_check_params () {
 		exit 1
 	fi
 
+	# Check that securelevel <= 0.  Otherwise we can't update schg files.
+	if [ `sysctl -n kern.securelevel` -gt 0 ]; then
+		echo "Updates cannot be installed when the system securelevel"
+		echo "is greater than zero."
+		exit 1
+	fi
+
 	# Check that we have a working directory
 	_WORKDIR_bad="Directory does not exist or is not writable: "
 	if ! [ -d "${WORKDIR}" -a -w "${WORKDIR}" ]; then
@@ -1356,7 +1363,12 @@ fetch_files_prepare () {
 
 	# List of files wanted
 	cut -f 2 -d '|' < $3.hashes |
-	    sort -u > files.wanted
+	    sort -u |
+	    while read HASH; do
+		if ! [ -f files/${HASH}.gz ]; then
+			echo ${HASH}
+		fi
+	done > files.wanted
 
 	# Generate a list of unmodified files
 	comm -12 $1.hashes $2.hashes |