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/syscall_darwin.go

@@ -31,6 +31,36 @@ type SockaddrDatalink struct {
 	raw    RawSockaddrDatalink
 }
 
+// SockaddrCtl implements the Sockaddr interface for AF_SYSTEM type sockets.
+type SockaddrCtl struct {
+	ID   uint32
+	Unit uint32
+	raw  RawSockaddrCtl
+}
+
+func (sa *SockaddrCtl) sockaddr() (unsafe.Pointer, _Socklen, error) {
+	sa.raw.Sc_len = SizeofSockaddrCtl
+	sa.raw.Sc_family = AF_SYSTEM
+	sa.raw.Ss_sysaddr = AF_SYS_CONTROL
+	sa.raw.Sc_id = sa.ID
+	sa.raw.Sc_unit = sa.Unit
+	return unsafe.Pointer(&sa.raw), SizeofSockaddrCtl, nil
+}
+
+func anyToSockaddrGOOS(fd int, rsa *RawSockaddrAny) (Sockaddr, error) {
+	switch rsa.Addr.Family {
+	case AF_SYSTEM:
+		pp := (*RawSockaddrCtl)(unsafe.Pointer(rsa))
+		if pp.Ss_sysaddr == AF_SYS_CONTROL {
+			sa := new(SockaddrCtl)
+			sa.ID = pp.Sc_id
+			sa.Unit = pp.Sc_unit
+			return sa, nil
+		}
+	}
+	return nil, EAFNOSUPPORT
+}
+
 // Some external packages rely on SYS___SYSCTL being defined to implement their
 // own sysctl wrappers. Provide it here, even though direct syscalls are no
 // longer supported on darwin.