MXS-1479 By default, refuse to run as root

MaxScale now refuses to run as root. However, it is possible to
start MaxScale as root, as long as a user to run MaxScale as is
provided as a command line argument.

It is possible to run as root by invoking MaxScale as root and
by specifying the MaxScale user to be root.

Documentation/Changelog.md

@@ -23,6 +23,7 @@
 * Binlog router supports MariaDB 10 GTID at both ends.
 * KILL CONNECTION can now be used through MaxScale.
 * Environment variables can now be used in the MaxScale configuration file.
+* By default, MaxScale can no longer be run as root.
 
 For more details, please refer to:
 * [MariaDB MaxScale 2.2.1 Release Notes](Release-Notes/MaxScale-2.2.1-Release-Notes.md)

Documentation/Release-Notes/MaxScale-2.2.1-Release-Notes.md

@@ -10,6 +10,21 @@ report at [Jira](https://jira.mariadb.org).
 
 ## Changed Features
 
+### Process identity
+
+By default, MaxScale can no longer be run as `root`, but must be run as some
+other user. However, it is possible to start MaxScale as `root`, as long as
+the user to run MaxScale as is provided as a command line argument:
+```
+root@host:~# maxscale --user=maxuser ...
+```
+If it is imperative to run MaxScale as root, e.g. in a Docker container, it
+can be achieved by invoking MaxScale as root and by explicitly specifying
+the user to also be root:
+```
+root@host:~# maxscale --user=root ...
+```
+
 ### Binlog server
 
 * The `mariadb10_slave_gtid` parameter was removed and slave connections can now