modules: use configurable poll interval

Make poll interval configurable. Use option from

----
config core 'main'
    option pollinterval '5'
---

This will only be used if the interval option is less or equal zero.
If the poll interval is not configured then a default value of 5 seconds
is used.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

modules/luci-base/htdocs/luci-static/resources/xhr.js

@@ -169,8 +169,8 @@ XHR.post = function(url, data, callback) {
 }
 
 XHR.poll = function(interval, url, data, callback, post) {
-	if (isNaN(interval) || interval < 1)
-		interval = 5;
+	if (isNaN(interval) || interval <= 0)
+		interval = L.env.pollinterval;
 
 	if (!XHR._q) {
 		XHR._t = 0;

modules/luci-base/luasrc/view/header.htm

@@ -18,6 +18,7 @@
 		resource    = resource,
 		scriptname  = luci.http.getenv("SCRIPT_NAME"),
 		pathinfo    = luci.http.getenv("PATH_INFO"),
-		requestpath = luci.dispatcher.context.requestpath
+		requestpath = luci.dispatcher.context.requestpath,
+		pollinterval    = luci.config.main.pollinterval or 5
 	}) %>);
 </script>