[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_rsp.c

@@ -853,7 +853,7 @@ rsp_request(struct evhttp_request *req)
   /* Check authentication */
   lib = cfg_getsec(cfg, "library");
   passwd = cfg_getstr(lib, "password");
-  if (passwd)
+  if (passwd && !cfg_getbool(cfg_getsec(cfg, "general"), "promiscuous_mode"))
     {
       libname = cfg_getstr(lib, "name");