Link.of_char: treatment of unknown link indicators

[reynir]

Since it was first introduced unknown-to-us link indicators are treated as normal files:

ocaml-tar/lib/tar.ml

Line 300 in d1c2689

| _ -> Normal (* if value is malformed, treat as a normal file *)

Now this affects reading headers since #129 where we synthesize directory link indicators for "normal" files whose file name end with a slash - this should really only be done for headers using '0' or '\000' as link indicator.

The question is what should we do about unknown-to-us link indicators? Options that come to mind are:

• Keep the status quo and pretend they are normal files (IMO undesirable),
• Add a Unknown of char constructor. This allows users to handle link indicators that are unknown to ocaml-tar. On the other hand it is a headache for writing as the user can construct e.g. Unknown '\000' which is not actually unknown to us (legacy Normal link indicator).
• Error out on unknown-to-us link indicators. This may mean we aren't able to read some archives (incorrectly or not). I'm not sure how widespread these archives are. I am unsure how likely it is to work to pretend an unknown link indicator is a normal file. I think I prefer this option although it is a breaking change.