portlist: add Poller.IncludeLocalhost option

This PR parameterizes receiving loopback updates from the portlist package.
Callers can now include services bound to localhost if they want.
Note that this option is off by default still.

Fixes #8171

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

portlist/poller.go

@@ -24,6 +24,11 @@ var debugDisablePortlist = envknob.RegisterBool("TS_DEBUG_DISABLE_PORTLIST")
 // Poller scans the systems for listening ports periodically and sends
 // the results to C.
 type Poller struct {
+	// IncludeLocalhost controls whether services bound to localhost are included.
+	//
+	// This field should only be changed before calling Run.
+	IncludeLocalhost bool
+
 	c chan List // unbuffered
 
 	// os, if non-nil, is an OS-specific implementation of the portlist getting
@@ -62,7 +67,7 @@ type osImpl interface {
 }
 
 // newOSImpl, if non-nil, constructs a new osImpl.
-var newOSImpl func() osImpl
+var newOSImpl func(includeLocalhost bool) osImpl
 
 var errUnimplemented = errors.New("portlist poller not implemented on " + runtime.GOOS)
 
@@ -100,7 +105,7 @@ func (p *Poller) setPrev(pl List) {
 
 func (p *Poller) initOSField() {
 	if newOSImpl != nil {
-		p.os = newOSImpl()
+		p.os = newOSImpl(p.IncludeLocalhost)
 	}
 }