MEDIUM: cli: show and change CLI permissions

Implement "show cli level" which show the level of the current CLI
session.

Implement "operator" and "user" which lower the permissions of the
current CLI session.
diff --git a/doc/management.txt b/doc/management.txt
index 03f742c..24a253f 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1804,6 +1804,31 @@
 show backend
   Dump the list of backends available in the running process
 
+show cli level
+  Display the CLI level of the current CLI session. The result could be
+  'admin', 'operator' or 'user'. See also the 'operator' and 'user' commands.
+
+  Example :
+
+    $ socat /tmp/sock1 readline
+    prompt
+    > operator
+    > show cli level
+    operator
+    > user
+    > show cli level
+    user
+    > operator
+    Permission denied
+
+operator
+  Decrease the CLI level of the current CLI session to operator. It can't be
+  increase. See also "show cli level"
+
+user
+  Decrease the CLI level of the current CLI session to user. It can't be
+  increase. See also "show cli level"
+
 show cli sockets
   List CLI sockets. The output format is composed of 3 fields separated by
   spaces. The first field is the socket address, it can be a unix socket, a