x/sys/unix: some SYS constants disappeared on Darwin

[bradfitz]

We updated our golang.org/x/sys dependency from v0.0.0-20200812155832 to v0.0.0-20201008064518-c1f3e3309c71 and got a build breakage on GOOS=darwin:

../../wireguard-go/tun/tun_darwin.go:141:3: undefined: unix.SYS_IOCTL
../../wireguard-go/tun/tun_darwin.go:162:3: undefined: unix.SYS_CONNECT
../../wireguard-go/tun/tun_darwin.go:241:4: undefined: unix.SYS_GETSOCKOPT
../../wireguard-go/tun/tun_darwin.go:346:3: undefined: unix.SYS_IOCTL
../../wireguard-go/tun/tun_darwin.go:380:3: undefined: unix.SYS_IOCTL

/cc @ianlancetaylor @tklauser

[bcmills]

git bisect reports CL 250437 (approved by @bradfitz)

[ianlancetaylor]

These were dropped because (as you know) macOS doesn't want us to make direct system calls anyhow. They want us to go through libSystem in all cases.

[gopherbot]

Change https://golang.org/cl/262959 mentions this issue: unix: add IoctlGetCtlInfo on darwin

[gopherbot]

Change https://golang.org/cl/264637 mentions this issue: unix: add IoctlIfreqMTU on darwin

[gopherbot]

Change https://golang.org/cl/264638 mentions this issue: unix: implement AF_SYSTEM/AF_SYS_CONTROL sockets on darwin

[tklauser]

The build breakage in wireguard-go should be addressed by WireGuard/wireguard-go#42