scripts: new log (#2493)

* added new `_log` function

With `_log`, the `_notify` method wa rendered obsolete. `_notify` was
not completely removed due to test failures in `check-for-changes.sh`.

The new `_log` function properly uses log levels such as `trace`,
`debug`, `info`, `warn` and `error`. It provides a cleaner solution
and renders `DMS_DEBUG` obsolete too (as only `_notify` depends on it).

* converted all helper script to new `_log` function

* converted all startup stacks to new `log` function

* `start-mailserver.sh` now uses new `_log` function

* final test and misc small script adjustments

* updated documentation

docs/content/config/environment.md

@@ -15,9 +15,17 @@ title: Environment Variables
 
 ##### DMS_DEBUG
 
+This environment variable is deprecated. Use `LOG_LEVEL` instead.
+
 - **0** => Debug disabled
 - 1     => Enables debug on startup
 
+##### LOG_LEVEL
+
+Set the log level for DMS. This is mostly relevant for container startup scripts and change detection event feedback.
+
+Valid values (in order of increasing verbosity) are: `error`, `warn`, `info`, `debug` and `trace`. The default log level is `info`.
+
 ##### SUPERVISOR_LOGLEVEL
 
 Here you can adjust the [log-level for Supervisor](http://supervisord.org/logging.html#activity-log-levels). Possible values are