Merge a change from MySQL:

  ------------------------------------------------------------
  revno: 2646.161.4
  committer: Tatiana A. Nurnberg <azundris@mysql.com>
  branch nick: 51-31177v2
  timestamp: Mon 2009-01-12 06:32:49 +0100
  message:
    Bug#31177: Server variables can't be set to their current values
    
    Bounds-checks and blocksize corrections were applied to user-input,
    but constants in the server were trusted implicitly. If these values
    did not actually meet the requirements, the user could not set change
    a variable, then set it back to the (wonky) factory default or maximum
    by explicitly specifying it (SET <var>=<value> vs SET <var>=DEFAULT).
    
    Now checks also apply to the server's presets. Wonky values and maxima
    get corrected at startup. Consequently all non-offsetted values the user
    sees are valid, and users can set the variable to that exact value if
    they so desire.