Allow use of absolute paths on SLFS

If the path starts with /, do not drop it.
There are no directories on SLFS, but use of /pretend/paths/to/files.txt
is common. What we do drop is the ./ prefix (added by mongoose when document_root=.)

PUBLISHED_FROM=5108bc078dfaf8f8afa4db554b4769d9d7b8a103