Split run.sh and add git retrival

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.

data/env.sh

@@ -0,0 +1,9 @@
+BORG_DATA_DIR=/backup
+SSH_KEY_DIR=/sshkeys
+BORG_CMD='cd ${BORG_DATA_DIR}/${client_name}; borg serve --restrict-to-path ${BORG_DATA_DIR}/${client_name} ${BORG_SERVE_ARGS}'
+AUTHORIZED_KEYS_PATH=/home/borg/.ssh/authorized_keys
+
+# Append only mode?
+BORG_APPEND_ONLY=${BORG_APPEND_ONLY:=no}
+
+export BORG_DATA_DIR SSH_KEY_DIR BORG_CMD AUTHORIZED_KEYS_PATH BORG_APPEND_ONLY