unix: add SysctlKinfoProc on darwin

This allows to get KinfoProc for a given process using
SysctlKinfoProcSlice("kern.proc.pid", pid) rather than having to query
all processes using SysctlKinfoProcSlice() and the extracting the
relevant KinfoProc.

Change-Id: I965ea5c77d6f3441592b4540c54ab56f6ac9e27d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/359676
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_darwin_test.go

@@ -256,3 +256,14 @@ func TestGetsockoptXucred(t *testing.T) {
 		}
 	}
 }
+
+func TestSysctlKinfoProc(t *testing.T) {
+	pid := unix.Getpid()
+	kp, err := unix.SysctlKinfoProc("kern.proc.pid", pid)
+	if err != nil {
+		t.Fatalf("SysctlKinfoProc: %v", err)
+	}
+	if got, want := int(kp.Proc.P_pid), pid; got != want {
+		t.Errorf("got pid %d, want %d", got, want)
+	}
+}