Change things around so that we add a row for each log file group that lists
the free space for that log file group.


client/mysqldump.c:
  dump logfile groups correctly (ignore row for the log file group itself)
mysql-test/r/ndb_dd_dump.result:
  update results with new row for log file groups
sql/ha_ndbcluster.cc:
  for fill_files_table:
  - set null bits properly
  - add rows for log file groups with free undo space.
sql/sql_show.cc:
  make most I_S.FILES columns nullable to better fit with reality.