bug#43535: File locking on Windows

[Eli Zaretskii]

> From: Richard Copley <rcopley@gmail.com>
> Date: Sun, 20 Sep 2020 16:54:56 +0100
> 
> This is quite a cruel punishment for an understandable mistake. I
> assume it started happening when Paul fixed the error handling in
> "filelock.c". It happens on master and Emacs 27.2, and not on Emacs
> 26.3.
> 
> There is more than one conceivable way to avoid this. E.g., use a
> different lock file name, keep the lock file open in deny-share mode
> to prevent accidents, ignore certain errors while unlocking.

I think ignoring errors, perhaps after asking for confirmation, is the
only way.  Using a different file name will just move the problem
elsewhere, and keeping the file open will disallow stealing the lock,
and on MS-Windows will have other annoying effects (like keeping the
file's directory busy etc.).