This patch modularize run.sh, adding two new helper scripts and
make it possible to specify a git repository for ssh keys via a
new env variable `BORG_SSHKEYS_REPO`.
the modularization add two new files :
- `env.sh` : define a few envriroment variables
- `create-client-dirs.sh` : update and create user directories and
re-create authorized_keys
We also add a new script `update-ssh-keys.sh` to be called regurlarly
in a cron job to check if the git repository is up-to-date and
eventually adding/removing users.