docs+tools: improve ndnsec help, error messages, and manual page

Exit with status code 2 on malformed command line

Refs: #3777
Change-Id: Id682a5d69c15c74cfa1903db38b5389936f5be58
diff --git a/docs/manpages/ndnsec-list.rst b/docs/manpages/ndnsec-list.rst
index bc4a316..a3de1a9 100644
--- a/docs/manpages/ndnsec-list.rst
+++ b/docs/manpages/ndnsec-list.rst
@@ -9,7 +9,7 @@
 
 ::
 
-    $ ndnsec-list [-h] [-KkCc]
+    ndnsec-list [-h] [-KkCc]
 
 Description
 -----------