clear HitEof flag in FileFd::Seek

clear HitEof flag in FileFd::Seek

fseek and co do this to their eof-flags and it is more logic this way as
we will usually seek away from the end (e.g. to re-read the file).

The commit also improves the testcase further and adds a test for the
binary compressor codepath (as gz, bzip2 and xz are handled by
libraries) via the use of 'rev' as a 'compressor'.