Fix #81223: flock() only locks first byte of file

`flock()` should lock the whole file, like on other systems which use
mandatory locking. We cannot use `0` like for `flck.l_len`, so we

Fix #81223: flock() only locks first byte of file

`flock()` should lock the whole file, like on other systems which use
mandatory locking. We cannot use `0` like for `flck.l_len`, so we use
the largest number, what is valid according to the documentation:
<https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-lockfileex#remarks>.

Closes GH-7216.

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