Proper detection of ephemeral ports on Mac OS (#7539)

Use systemctl to properly detect ephemeral ports on Mac OS (aka darwin) by fetching
systemctl values:
 * net.inet.ip.portrange.first
 * net.inet.ip.portrange.last

This will avoid the message:

`[INFO] freeport: ephemeral port range detection not configured for GOOS="darwin"`

and properly detect the correct port range

sdk/freeport/ephemeral_darwin.go

@@ -0,0 +1,37 @@
+//+build darwin
+
+package freeport
+
+import (
+	"fmt"
+	"os/exec"
+	"regexp"
+	"strconv"
+)
+
+const ephemeralPortRangeSysctlFirst = "net.inet.ip.portrange.first"
+const ephemeralPortRangeSysctlLast = "net.inet.ip.portrange.last"
+
+var ephemeralPortRangePatt = regexp.MustCompile(`^\s*(\d+)\s+(\d+)\s*$`)
+
+func getEphemeralPortRange() (int, int, error) {
+	cmd := exec.Command("/usr/sbin/sysctl", "-n", ephemeralPortRangeSysctlFirst, ephemeralPortRangeSysctlLast)
+	out, err := cmd.Output()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	val := string(out)
+
+	m := ephemeralPortRangePatt.FindStringSubmatch(val)
+	if m != nil {
+		min, err1 := strconv.Atoi(m[1])
+		max, err2 := strconv.Atoi(m[2])
+
+		if err1 == nil && err2 == nil {
+			return min, max, nil
+		}
+	}
+
+	return 0, 0, fmt.Errorf("unexpected sysctl value %q for keys %q, %q", val, ephemeralPortRangeSysctlFirst, ephemeralPortRangeSysctlLast)
+}

sdk/freeport/ephemeral_darwin_test.go

@@ -0,0 +1,18 @@
+//+build darwin
+
+package freeport
+
+import (
+	"testing"
+)
+
+func TestGetEphemeralPortRange(t *testing.T) {
+	min, max, err := getEphemeralPortRange()
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if min <= 0 || max <= 0 || min > max {
+		t.Fatalf("unexpected values: min=%d, max=%d", min, max)
+	}
+	t.Logf("min=%d, max=%d", min, max)
+}

sdk/freeport/ephemeral_fallback.go

@@ -1,4 +1,4 @@
-//+build !linux
+//+build !linux,!darwin
 
 package freeport