Fix behaviour of dynamicdhcp field (thanks to Fabio Mercuri)

modules/admin-full/luasrc/model/cbi/admin_network/dhcp.lua

@@ -45,7 +45,11 @@ s:option(Value, "limit", translate("limit")).rmempty = true
 
 s:option(Value, "leasetime").rmempty = true
 
-s:option(Flag, "dynamicdhcp").rmempty = true
+local dd = s:option(Flag, "dynamicdhcp")
+dd.rmempty = false
+function dd.cfgvalue(self, section)
+	return Flag.cfgvalue(self, section) or "1"
+end
 
 s:option(Value, "name", translate("name")).optional = true
 
@@ -65,4 +69,4 @@ for i, n in ipairs(s.children) do
 	end
 end
 
-return m
+return m