Stop trying to access filesystems that are no longer in dynamically generated conky text

[quiescens]

Checklist

• [x] I have described the changes
• [x] I have linked to any relevant GitHub issues, if applicable
• [ ] Documentation in doc/ has been updated
• [x] All new code is licensed under GPLv3

Description

Dynamically updated conky texts that use "fs" variables can leave their entries in fs_stats after they have been removed from the text dynamically.

Instead of having update_fs_stats() call update_fs_stat() for all entries that have the "set" flag, just clear the "set" flag. This will "remove" the current fs_stats entries (including in particular, filesystems that no longer appear in the conky text).

On the next update cycle, prepare_fs_stat() will call update_fs_stat() for anything that requires it, and will retain the logic that prevents it from updating the same fs multiple times.

Note: Depending on how well update frequencies line up, it is still possible to get stat errors but they won't continue forever.

• fixes #208 hopefully properly
• also fixes the niche but possible issue of exhausting the total fs_stats entries if someone adds and removes many "fs" targets over time (eg. a long running system that has many USB drives plugged and unplugged)
• as far as I can tell this does not increase the frequency of attempted stat calls, and should have very minimal effect on overall load

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