Add support for dragonfly bsd.

Simply copy the pse implementation for openbsd.

server/pse/pse_dragonfly.go

@@ -0,0 +1,36 @@
+// Copyright 2015-2023 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Copied from pse_openbsd.go
+
+package pse
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+// ProcUsage returns CPU usage
+func ProcUsage(pcpu *float64, rss, vss *int64) error {
+	pidStr := fmt.Sprintf("%d", os.Getpid())
+	out, err := exec.Command("ps", "o", "pcpu=,rss=,vsz=", "-p", pidStr).Output()
+	if err != nil {
+		*rss, *vss = -1, -1
+		return fmt.Errorf("ps call failed:%v", err)
+	}
+	fmt.Sscanf(string(out), "%f %d %d", pcpu, rss, vss)
+	*rss *= 1024 // 1k blocks, want bytes.
+	*vss *= 1024 // 1k blocks, want bytes.
+	return nil
+}