r475: * dh_gencontrol: Added a documented interface for specifying substvars

r475:    * dh_gencontrol: Added a documented interface for specifying substvars
     data in a file. Substvars data may be put in debian/package.substvars.
     (Those files used to be used by debhelper for automatically generated
     data, but it uses a different internal filename now). It will be merged
     with any automatically determined substvars data. See bug #98819
   * I want to stress that no one should ever rely in internal, undocumented
     debhelper workings. Just because debhelper uses a certian name for some
     internally used file does not mean that you should feel free to modify
     that file to your own ends in a debian package. If you do use it, don't
     be at all suprised when it breaks. If you find that debhelper is lacking
     a documented interface for something that you need, ask for it!
     (debhelper's undocumented, internal use only files should now all be
     prefixed with ".debhelper")