Improvements to mtr

- Added option $backup_on_restart to store a backup of the var/
  directory after restart. This is useful for debugging recovery.
- The list_files option in mysqltest is now returning the number of
  found files in $sys_files. This is useful to break test if there are
  unknown or lost files in the data directory.

mysql-test/include/restart_mysqld.inc

@@ -2,11 +2,17 @@
 #
 # [--let $shutdown_timeout= 60]
 # [--let $allow_rpl_inited= 1]
+# Create a copy of the var directory after restart
+# [--let $$backup_on_restart=1]
 # --source include/restart_mysqld.inc
 
 --source include/not_embedded.inc
 
 --source include/shutdown_mysqld.inc
+if ($backup_on_restart)
+{
+  --exec /bin/tar cfzP /tmp/mtr-backup.tgz --exclude=*.sock $MYSQLTEST_VARDIR/*
+}
 --source include/start_mysqld.inc
 
 # The following sleep is required to give sleep_until_file_created() time