Add load averages to OS stats on FreeBSD

This commit adds load averages to the OS stats on FreeBSD. For these
stats to be available, linprocfs must be available and mounted at
/compat/linux/proc.

core/src/main/java/org/elasticsearch/monitor/os/OsProbe.java

@@ -111,8 +111,9 @@ public class OsProbe {
      * Returns the system load averages
      */
     public double[] getSystemLoadAverage() {
-        if (Constants.LINUX) {
-            double[] loadAverage = readProcLoadavg("/proc/loadavg");
+        if (Constants.LINUX || Constants.FREE_BSD) {
+            final String procLoadAvg = Constants.LINUX ? "/proc/loadavg" : "/compat/linux/proc/loadavg";
+            double[] loadAverage = readProcLoadavg(procLoadAvg);
             if (loadAverage != null) {
                 return loadAverage;
             }

core/src/main/resources/org/elasticsearch/bootstrap/security.policy

@@ -118,4 +118,7 @@ grant {
 
   // load averages on Linux
   permission java.io.FilePermission "/proc/loadavg", "read";
+
+  // load averages on FreeBSD
+  permission java.io.FilePermission "/compat/linux/proc/loadavg", "read";
 };