docs(install): and info to debug
refactor(mk): use 'sed' to prepend string to /etc/hosts
style(service): add sectionning comments
refactor(service): declare path to pids directory in '$PID_DIR'
style(service): format doc comments
style(service): add 'function' keyword to declaration
refactor(service): add mechanism to apply to all or only specified config

We now pass a config name to start|stop|status instead of a config path.
Most of the variable have now a local scope.

* iter_action_on_config(): List available configuration in $CONFIG_DIR
* list_config(): Execute action on given configurations
refactor(service): declare as local variable
style(service): improve formatting for clarity
feat(service): add a debug flag mechanism

To debug the service you will need to export the `DEBUG` variable with a non-empty value and tell `sudo`
 to preserve environment with `-e` or --preserve-env` option. So it can the exported variable is available to `sudo` context.

 ````bash
 export DEBUG="true"
 sudo -E /etc/init.d/mast status
 ```
refactor(service): define 'autossh_pid' only when there is a pid file
style(service): move comment
refactor(service): fallback to default return status
fix(service): toggle off 'errexit' option (too obscure to debug)
style(service): typo
style(service): typo
fix(service): protect string expansion
feat(service,start): verify if pid exists
docs(installation): unset DEBUG
fix(service,status): protect string expansion