all: leaking queries to OS resolver instead of SRVFAIL

So it would work in more general case than just captive portal network,
which ctrld have supported recently.

Uses who may want no leaking behavior can use a config to turn off this
feature.

docs/config.md

@@ -281,6 +281,13 @@ The value must be a positive number, any invalid value will be ignored and defau
 - Required: no
 - Default: 3600
 
+### leak_on_upstream_failure
+Once ctrld is "offline", mean ctrld could not connect to any upstream, next queries will be leaked to OS resolver.
+
+- Type: boolean
+- Required: no
+- Default: true on Windows, MacOS and non-router Linux.
+
 ## Upstream
 The `[upstream]` section specifies the DNS upstream servers that `ctrld` will forward DNS requests to.