net/interfaces: add State.PAC field, populate it on Windows

Not used for anything yet (except logging), but populate the current
proxy autoconfig PAC URL in Interfaces.State.

A future change will do things based on it.

net/interfaces/interfaces.go

@@ -177,6 +177,9 @@ type State struct {
 
 	// HTTPProxy is the HTTP proxy to use.
 	HTTPProxy string
+
+	// PAC is the URL to the Proxy Autoconfig URL, if applicable.
+	PAC string
 }
 
 func (s *State) Equal(s2 *State) bool {
@@ -197,6 +200,9 @@ func (s *State) RemoveTailscaleInterfaces() {
 	}
 }
 
+// getPAC, if non-nil, returns the current PAC file URL.
+var getPAC func() string
+
 // GetState returns the state of all the current machine's network interfaces.
 //
 // It does not set the returned State.IsExpensive. The caller can populate that.
@@ -222,6 +228,9 @@ func GetState() (*State, error) {
 	if u, err := tshttpproxy.ProxyFromEnvironment(req); err == nil && u != nil {
 		s.HTTPProxy = u.String()
 	}
+	if getPAC != nil {
+		s.PAC = getPAC()
+	}
 
 	return s, nil
 }