checkPoFiles function added to R-devel

I've just added a couple of functions to the tools package to check for 
bad format strings in the .po files that translators produce.  See 
?tools::checkPoFiles for the docs.  They detect cases where the 
translation has different format conversions than the original English 
message.  This isn't always an error, but it often is.

Hopefully this will make it easier to detect problems before they're 
released.

Duncan Murdoch