unix: add ptrace(PT_DENY_ATTACH) wrapper for darwin

Change-Id: I13e4426e6e411ceb449539479d9bcee15a518cf6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/464655
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

Author: tmthrgd

unix/syscall_darwin.go

@@ -230,6 +230,7 @@ func direntNamlen(buf []byte) (uint64, bool) {
 
 func PtraceAttach(pid int) (err error) { return ptrace(PT_ATTACH, pid, 0, 0) }
 func PtraceDetach(pid int) (err error) { return ptrace(PT_DETACH, pid, 0, 0) }
+func PtraceDenyAttach() (err error)    { return ptrace(PT_DENY_ATTACH, 0, 0, 0) }
 
 //sysnb	pipe(p *[2]int32) (err error)