Prevent seek_in from marking buffer data as valid after closing the channel (Fixes #11878)

[shym]

This proposes a fix for issue #11878 in which some part of a in_channel buffer can be considered as valid after the channel has been closed.
This works by simply resetting the channel offset to 0 when the channel is closed to disable any backward seek_in later.
This PR also introduces a test for that issue (before and with the fix).

Closes issue #11878

[OlivierNicole]

The test you introduced fails on Windows. Maybe lseek behaves differently there?

[xavierleroy]

Better use open_in_bin instead of open_in in the test. Seeking on files opened in text mode under Windows generally doesn't do what you want.

[shym]

Thank you for the tip!
I hope I correctly handled the other CI remarks while I was updating the branch.

[xavierleroy]

Looks good to me. The change makes complete sense and there's a test.

[gasche]

I'm trying to understand this weird offset = 0 assignment when closing channels in the context of #12898.

Naive question: why don't we fix the present issue by failing on seek_in on a closed channel? Is there any issue with that approach?
There is a fast path in caml_seek_in, where the position we seek is available in the input buffer, I propose to fail in that case (as in the other case) when the file descriptor is invalid.

This requires changing the test introduced in the present PR,

  let ic = open_in_bin Sys.argv.(0) in
  seek_in ic nb_bytes;
  close_in ic;
  seek_in ic 0;
  assert (
    try
      ignore (input_byte ic);
      false
    with
    | Sys_error _ -> true
    | _           -> false)

to move the seek_in ic 0 inside the try .. with block, because now the code fails at seek_in time rather than at input_byte.