map-split: use relative instead of absolute path for symlinks

[unode]

As part of the split-map strategy implemented in ngless 0.6 symlinks are created to avoid copying split files. As of this version symlinks use absolute paths. While this works on a single machine it may break on shared filesystems (different mountpoints) or if the index folder needs to be manually relocated.

Making symlinks relative to the root of the index folder, addresses the issues mentioned above.