Small change to warn about symlinks pointing outside of a backup

[MartinKurtz]

Hi, this is related to #9254, and should implement the most basic version of "the easy option" outlined there.

It implements a warning about symlinks pointing outside of the backup mountpoint when using borg mount and adds it to the borg mount documentation as well as logs such a warning on a mount call.

I will mark this as a draft as i see this more as a preliminary suggestion.

[ThomasWaldmann]

Thanks for the PR. Good idea to have this as an easy/quick fix until we might have something better.

[MartinKurtz]

I think i am happy with the wordings now

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.57%. Comparing base (4eed3ec) to head (2da7c91).
⚠️ Report is 1 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9255      +/-   ##
=============================================
- Coverage      80.59%   80.57%   -0.02%     
=============================================
  Files             38       38              
  Lines          11256    11256              
  Branches        1771     1771              
=============================================
- Hits            9072     9070       -2     
- Misses          1615     1616       +1     
- Partials         569      570       +1     

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