[misc] fix editing of resource file with VS2012

* Fix "warning RC4005: '__useHeader': redefinition" that prevents
  opening the RC file in Visual Studio 2012 when XP mode is enabled.
[loc] misc. fixes and improvements

* Add support for LCID provision to option -l
* Fix French translation
* Fix memory leak for loc_filename
* Don't delete an external 'rufus.loc' file if present
* Use IETF normalized tags
* Add quick instructions and sample for translators in new_translation.loc
* Improve About dialog