gluon-wan-dnsmasq: don't rewrite resolv.conf when its content hasn't changed

This avoids poking dnsmasq every time a DHCP lease is obtained or a router
advertisement received.

package/gluon-wan-dnsmasq/files/etc/config/gluon-wan-dnsmasq

 config 'static'
-       # list 'server' '192.168.0.1'		# Example
+	# list 'server' '192.168.0.1'		# Example

package/gluon-wan-dnsmasq/files/lib/gluon/wan-dnsmasq/update.lua

@@ -9,27 +9,38 @@ local uci = require('luci.model.uci').cursor()
 local fs = require 'nixio.fs'
 
 
-local function write_servers(f, servers)
+local new_servers = ''
+
+
+local function append_servers(servers)
   for _, server in ipairs(servers) do
-    f:write('nameserver ', server, '\n')
+    new_servers = new_servers .. 'nameserver ' .. server .. '\n'
   end
 end
 
-local function write_interface_servers(f, iface)
-  write_servers(f, ubus:call('network.interface.' .. iface, 'status', {}).inactive['dns-server'])
+local function append_interface_servers(iface)
+  append_servers(ubus:call('network.interface.' .. iface, 'status', {}).inactive['dns-server'])
 end
 
 
-fs.mkdirr(RESOLV_CONF_DIR)
-local f = io.open(RESOLV_CONF, 'w+')
-
 local static = uci:get_first('gluon-wan-dnsmasq', 'static', 'server')
 
 if type(static) == 'table' and #static > 0 then
-  write_servers(f, static)
+  append_servers(static)
 else
-  pcall(write_interface_servers, f, 'wan6')
-  pcall(write_interface_servers, f, 'wan')
+  pcall(append_interface_servers, 'wan6')
+  pcall(append_interface_servers, 'wan')
 end
 
-f:close()
+
+fs.mkdirr(RESOLV_CONF_DIR)
+
+local old_servers = fs.readfile(RESOLV_CONF)
+
+if new_servers ~= old_servers then
+   local f = io.open(RESOLV_CONF .. '.tmp', 'w')
+   f:write(new_servers)
+   f:close()
+
+   fs.rename(RESOLV_CONF .. '.tmp', RESOLV_CONF)
+end