Change absolute binary paths in scripts to /usr/bin/env or make them
relative.
This allows the scripts to be used on linux distributions
like NixOS, where binaries are not located in /usr/ or /bin/.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
