internal/netlink: expose multicast groups

This also gets rid of the cached pid value for port since that prevents multiple sockets from being open at once.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/netlink/netlink_test.go

@@ -5,8 +5,6 @@ import (
 	"testing"
 )
 
-func init() { nlPidOnce.Do(func() {}); nlPid = 1 }
-
 type payloadTestCase struct {
 	name string
 	f    func(c *conn)
@@ -21,8 +19,9 @@ func checkPayload(t *testing.T, testCases []payloadTestCase) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
+			t.Helper()
 
-			c := conn{pos: syscall.NLMSG_HDRLEN}
+			c := conn{port: 1, pos: syscall.NLMSG_HDRLEN}
 			tc.f(&c)
 			if got := c.pending(); string(got) != string(tc.want) {
 				t.Errorf("pending: %#v, want %#v", got, tc.want)