samba-tool domain backup: cope better with dangling symlinks

samba-tool domain backup currently crashes when it tries to backup a symlink to a non-existent file. With these two patches, we first make it not crash, then make it not try to follow the symlink at all.

The second patch will change the behaviour when Samba directories contain symlinks to real files: they will now not be included (symlinks to directories were never followed).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14918

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• (optional) This MR is just one part towards a larger feature.
• (optional, if backport required) Bugzilla bug filed and BUG: tag added
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code