net: `WriteMsgUDPAddrPort` should accept zero value destination addresses [1.25 backport]

[gopherbot]

@database64128 requested issue #74841 to be considered for backport to the next 1.25 minor release.

@gopherbot please open backport issues. Now this is a tough one compared to #74737.

In user code, the same code path usually needs to handle both connected and unconnected sockets. There's no workaround other than checking and not using WriteMsgUDPAddrPort when the address is zero value. This is quite ugly.

n, cmsgn, err := c.WriteMsgUDPAddrPort(b, cmsg, ap)
if err != nil {}

becomes:

var (
	n     int
	cmsgn int
	err   error
)
// Long explanation here...
if ap.IsValid() {
	n, cmsgn, err = c.WriteMsgUDPAddrPort(b, cmsg, ap)
} else {
	n, cmsgn, err = c.WriteMsgUDP(b, cmsg, nil)
}
if err != nil {}

The fix involves 3 separate CLs, touching internal/poll and syscall. Changes in these packages are mostly adding nil checks though, so risks should be low.

[dmitshur]

We discussed this in a release meeting. The bug has existed since Go 1.18, so it's not very new. Since it's possible to work around it, it seems anyone running into it may have done that. We agreed not to proceed with the backport; the fix will be be available in Go 1.26. Thanks.