Improve variable detection in checkvariables.php

```
/%([0-9]+\$){0,1}S/i
```
This also detects %s or %S.
pro: you can find more errors ([example for cs](http://transvision.mozfr.org/?recherche=browser%2Fchrome%2Fbrowser%2Fdevtools%2Fappcacheutils.properties%3AescapeSpaces&repo=central&sourcelocale=en-US&locale=cs&search_type=strings_entities)).
neg: more false positives. Code now reports a mismatch between %S and %s, and %S and %1$S, and both are correct. The alternative is to put more logic in the class

```
/(?<!%[0-9])\$[a-z0-9\.]+\b/i
```
This now matches only "brandshortname" in "test brandshortname.". Negative lookbehind is used to avoid some false positive, example ```My%1$SFeeds-%2$S.opml```.