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/portlist_linux_test.go

@@ -89,7 +89,7 @@ func TestParsePorts(t *testing.T) {
 			if tt.file != "" {
 				file = tt.file
 			}
-			li := newLinuxImplBase()
+			li := newLinuxImplBase(false)
 			err := li.parseProcNetFile(r, file)
 			if err != nil {
 				t.Fatal(err)
@@ -118,7 +118,7 @@ func BenchmarkParsePorts(b *testing.B) {
 		contents.WriteString("   3: 69050120005716BC64906EBE009ECD4D:D506 0047062600000000000000006E171268:01BB 01 00000000:00000000 02:0000009E 00000000  1000        0 151042856 2 0000000000000000 21 4 28 10 -1\n")
 	}
 
-	li := newLinuxImplBase()
+	li := newLinuxImplBase(false)
 
 	r := bytes.NewReader(contents.Bytes())
 	br := bufio.NewReader(&contents)