detect compressed status files on extension again

It handy to be able to point apt at reading a compressed dpkg/status
file in debugging cases, which worked pre-1.1 but somewhere down the
line in the massive refactoring. Restoring this behavior in a central
place for all realfile index files instead of just for the status file.

(This has no effect on index files acquired from an archive – those are
handled by different classes and support compressed files just fine)

apt-pkg/indexfile.cc

@@ -310,7 +310,7 @@ std::string pkgDebianIndexRealFile::GetProgressDescription() const
 }
 bool pkgDebianIndexRealFile::OpenListFile(FileFd &Pkg, std::string const &FileName)/*{{{*/
 {
-   if (Pkg.Open(FileName, FileFd::ReadOnly, FileFd::None) == false)
+   if (Pkg.Open(FileName, FileFd::ReadOnly, FileFd::Extension) == false)
       return _error->Error("Problem opening %s",FileName.c_str());
    return true;
 }