Fix Darwnin _lp_cpu_load()

With the previous code I get an error before each prompt:
-bash: $/dev/fd/62: No such file or directory

Maybe the same code can't be used for both Darwnin and FreeBSD.
I reuse the code removed in 2a833475bc6661a59b5eecc4f8aa343f4bb1977b

liquidprompt

@@ -161,13 +161,20 @@ case "$LP_OS" in
             echo "$load"
         }
         ;;
-    FreeBSD|Darwin)
+    FreeBSD)
         _lp_cpu_load () {
             local bol load eol
             read bol load eol < $<( LANG=C sysctl -n vm.loadavg )
             echo "$load"
         }
         ;;
+    Darwin)
+        _lp_cpu_load () {
+            local load
+            load=$(LANG=C sysctl -n vm.loadavg | awk '{print $2}')
+            echo "$load"
+        }
+        ;;
     SunOS)
         _lp_cpu_load () {
             LANG=C uptime | awk '{print substr($10,0,length($10))}'