[httpd] Add promiscuous mode option, i.e. no auth requirements

For people who want to avoid the web login, or want to get around pairing
problems. This is also added because commit #e59a1a1 means that all
Remotes are now subject to auth, not just those with a user-agent name that
starts with "Remote".

src/httpd.c

@@ -954,6 +954,9 @@ httpd_admin_check_auth(struct evhttp_request *req)
   const char *passwd;
   int ret;
 
+  if (cfg_getbool(cfg_getsec(cfg, "general"), "promiscuous_mode"))
+    return true;
+
   passwd = cfg_getstr(cfg_getsec(cfg, "general"), "admin_password");
   if (passwd)
     {