Rewrite absolute symlinks to stay inside their origin

Absolute symlinks have a different meaning when accessed though the regular filesystem, they don't anchor to the mounted URL anymore:

sftp://server/tmp/symlink -> /home/user/
/mnt/sftp/server/tmp/symlink -> /home/user/

Rewrite absolute symlinks when reading or writing them to keep the target consistent:

/mnt/sftp/server/tmp/symlink -> /mnt/sftp/server/home/user/

BUG: 422090

Todo:

• Actually test this
• Adjust existing test cases which fail now
• Add new test cases