bug#63627: Improve plstore.el and fix various issues of it

[Jens Schmidt]

On 2023-09-08  13:34, Eli Zaretskii wrote:
>> Date: Fri, 8 Sep 2023 11:16:50 +0200
>> From: Jens Schmidt <jschmidt4gnu@vodafonemail.de>
>> Cc: 63627@debbugs.gnu.org
>>
>> Independently of the tests, I'll do the Emacs 29 bugs next, also on this
>> bug.
> 
> Thanks.

After some consideration actually only one Emacs 29 bug requires fixing.
Namely that the sequence:

    (setq plstore (plstore-open plstore-test-file))
    (plstore-save plstore)
    (plstore-close plstore)

silently drops any encrypted data in PLSTORE.  Verified in my local
master with the new ERT tests, and also verified that the attached patch
fixes the issue.

Please consider for committing on emacs-29.

Thanks

0001-Fix-loss-of-encrypted-data-in-plstore.el.patch
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