We open for writing a known location, which is exploitable with a symlink
attack.  Now, use the EXCLusive flag, so that the presence of anything at 
that location causes a failure.  Try once to open safely, and if failure 
then remove that location and try again to open safely.  If both fail, then
raise an error.


client/mysql_upgrade.c:
  Open the file with the O_EXCL flag, so that a symlink attack would not work.
  
  If opening it fails, try removing something at that location, and try again.
  If the second time fails, then abort as previous.