#1681 - Copy-edits on MySQL version notices

* The titles for PHP+MySQL notices should use the same
  capitalization/hyphenation ("Out-of-Date" vs "Out of date").
* Extract some more "%n" variables. Fix one errant variable.
* Reduce floweriness of the "version" / "at least" / "or later" text.
* Make it possible to add/remove the "+" notation without changing
  the ts() message.