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

mailserver.env

@@ -13,10 +13,18 @@
 # => Specify a fully-qualified domainname to serve mail for.  This is used for many of the config features so if you can't set your hostname (e.g. you're in a container platform that doesn't let you) specify it in this environment variable.
 OVERRIDE_HOSTNAME=
 
+# (deprecated: use LOG_LEVEL instead)
 # 0 => Debug disabled
 # 1 => Enables debug on startup
 DMS_DEBUG=0
 
+# 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`.
+LOG_LEVEL=info
+
 # critical => Only show critical messages
 # error => Only show erroneous output
 # **warn** => Show warnings