Various Bugfixes and Improved Caching

• fix read() with offset especially with large files. This fixes use of
  various tools when applied on oscfs files e.g. rpm2cpio or
  tail *.spec previously failed or behaved strangely.
• be robust against non-ASCII characters in OBS buildlogs.
• apply caching also to actual file content to be more efficient when e.g.
  operating on larger files.