cubicweb #1903266 [bfss] fsimport should use symlinks in etc/cw.d/bfss [open]

rather than storing the direct file path
this may help avoid nastyness with db-dump/db-restore
(overall I'm a bit nervous about cw handling my photo files)

to be discussed

prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>