Backup persistence.conf before modifying it in t-p-s

… as suggested in #10976-note_27.

Even with the restore code suggested there, saving a backup should already be a net win:

• for users who might notice the “.bak” file and recover themselves
• for help desk when they help users recover from #10976
• for me when I get debugging data about #10976 (depending on the mtime of the backup file, I’ll be able to tell for sure if t-p-s was used at all recently, regardless of what users remember: nothing else modifies this backup file).

Feature Branch: bugfix/16461-lost-persistence-config, persistence-setup:bugfix/16461-lost-persistence-config

Parent Task: #10976

Related issues

• Related to #14544
• Related to #16568 (closed)
• Blocks #15507 (closed)

Original created by @intrigeri on 16461 (Redmine)