improved doc, added missing headers, file manipulation error management improved

modules/install_mkagent.sh

@@ -25,7 +25,7 @@ install_mkagent()
 precheck_install_mkagent()
 {
     [[ -n $MK_SERVER ]] && (
-        prnt E "Serveur check_mk non déclaré."
+        prnt E "Undeclared check_mk server."
         die 162
     )
 }