Introduce Index mount mode

[piste-jp]

Summary of changes

Introduce a feature to mount LTFS with an index file. And also a few improvements are provided.

Description

Introduce a feature to mount LTFS with an index file. The feature includes

1. Mount a tape with an index file and tape, contents of tape can be accessible on read-only
2. Mount a tape only with index, meta-data in the index file can be accessible

When both -o devname and -o rollback_mount=[file_name] is specified, the ltfs command tries to mount a tape by normal read-only mode with an index and a tape. User can read the contents of files.

When only -o rollback_mount=[file_name] is specified, the ltfs command tries to mount a tape by meta-dta read-only mode only with an index. User shall have EAGAIN when they try to read the contents of files.

In addition, a few functions and improvements are introduced too.

• Change -o rollback_mount option in ltfs to have a generation number or an index file to mount
• Change --capture-index option in ltfs and ltfsck to have a destination directory
• Improve the performance of capturing index

Type of change

Please delete items that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have confirmed my fix is effective or that my feature works