unix: implement AF_SYSTEM/AF_SYS_CONTROL sockets on darwin

See https://developer.apple.com/documentation/kernel/sockaddr_ctl for
details.

AF_SYSTEM is darwin-specific, so the BSD-specific anyToSockaddr func
needs to call GOOS-specific funcs which return nil, EAFNOSUPPORT on all
BSD platforms but darwin.

For golang/go#41868

Change-Id: Id7794aba5988dd9511053b76be5efeb66c104a26
Reviewed-on: https://go-review.googlesource.com/c/sys/+/264638
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/zerrors_darwin_arm64.go

@@ -45,6 +45,7 @@ const (
 	AF_SIP                            = 0x18
 	AF_SNA                            = 0xb
 	AF_SYSTEM                         = 0x20
+	AF_SYS_CONTROL                    = 0x2
 	AF_UNIX                           = 0x1
 	AF_UNSPEC                         = 0x0
 	AF_UTUN                           = 0x26