conn: fix StdNetBind fallback on Windows

If RIO is unavailable, NewWinRingBind() falls back to StdNetBind.
StdNetBind uses x/net/ipv{4,6}.PacketConn for sending and receiving
datagrams, specifically via the {Read,Write}Batch methods.
These methods are unimplemented on Windows and will return runtime
errors as a result. Additionally, only Linux benefits from these
x/net types for reading and writing, so we update StdNetBind to fall
back to the standard library net package for all platforms other than
Linux.

Reviewed-by: James Tucker <james@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/bind_std_test.go

@@ -0,0 +1,22 @@
+package conn
+
+import "testing"
+
+func TestStdNetBindReceiveFuncAfterClose(t *testing.T) {
+	bind := NewStdNetBind().(*StdNetBind)
+	fns, _, err := bind.Open(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	bind.Close()
+	buffs := make([][]byte, 1)
+	buffs[0] = make([]byte, 1)
+	sizes := make([]int, 1)
+	eps := make([]Endpoint, 1)
+	for _, fn := range fns {
+		// The ReceiveFuncs must not access conn-related fields on StdNetBind
+		// unguarded. Close() nils the conn-related fields resulting in a panic
+		// if they violate the mutex.
+		fn(buffs, sizes, eps)
+	}
+}