lib: fix more format warnings (#637)

The following patch was also neccessary to compile.

* command.c: (config_logmsg_cmd) use "%s" format spec
* if.c: (connected_log) ditto
diff --git a/lib/command.c b/lib/command.c
index 45eb9e3..ebb50b4 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -3054,7 +3054,7 @@
   if ((level = level_match(argv[0])) == ZLOG_DISABLED)
     return CMD_ERR_NO_MATCH;
 
-  zlog(NULL, level, ((message = argv_concat(argv, argc, 1)) ? message : ""));
+  zlog(NULL, level, "%s", ((message = argv_concat(argv, argc, 1)) ? message : ""));
   if (message)
     XFREE(MTYPE_TMP, message);
   return CMD_SUCCESS;