Handle read-only combination of tape and drive correctly like LTO5 tape
and LTO7 drive. I enbugged this corruption from 2.2.2.0 to 2.4.0.0.

The fix of issue #33.

Fix to print plugin help correctly. It is broken when teh tape backend name prefix from "driver" to "tape".

This fix is a part of the freebsd patch provided by Spectra Logic. I will backport this portion to v2.4.0 branch. The freebsd patch will be merged soon in master branch.

* Replace copyright year on v2.4.0 branch to 2018

This fix includes following changes or fixes.

    Accept an index created by bad manner index writers
    Fix the index writer to encode '%' correctly

This change is additional fix of #105.

Current LTFS rejects a US in name so need to escase US also as same as
'/'.

Avoid to recover with prior index when LTFS found a bad manner index.

The worst case is removing a good file in a bad index wihout any
warning.

- Stop seeking index when following error happens
  - memory shortage
  - parse error because of unexpected filename
  - parse error because of non UTF-8 characters
  - parse error because of too long filename