conf: do not warn about missing user directive unless root

there's no point in printing a warning if the program is already started
as a restricted user.

src/conf.c

@@ -505,7 +505,7 @@ int reload_config_file (const char *config_fname, struct config_s *conf)
                 goto done;
         }
 
-        if (!conf->user) {
+        if (!conf->user && !geteuid()) {
                 log_message (LOG_WARNING, "You SHOULD set a UserName in the "
                              "config file. Using current user instead.");
         }