Rename old 'xml' file as 'bak' file when saving

If you use the standard naming convention and end your subsurface
filename in ".xml", we will now save away any previous xml file as a
"bak" file before writing a new one.

This can be useful for:

- recovering from mistakes that deleted old dives

- seeing what changed (ie you can do things like "diff -u xyz.bak
  xyz.xml") after doing some operation and saving the result.

However, this does only a single level of backups - if you save twice,
you will obviously have lost the original. I'd strongly encourage some
external backup system in addition to this very simplistic backup.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

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