Use the sum of MemFree, Cached, and Buffers as real available memory

to improve the absence of MemAvailable field in old linux kernel version(prior to 3.14)
or to improve the incorrect MemAvailable in docker containers.
This PR solves issue processone#376

src/tsung_controller/ts_os_mon_erlang.erl

@@ -262,25 +262,11 @@ get_os_data(DataName) -> get_os_data(DataName,os:type()).
 %% Use the result of the free commands on Linux and os_mon on all
 %% other platforms
 get_os_data(freemem, {unix, linux}) ->
-    case file:open("/proc/meminfo",[raw,read]) of
-        {ok, FD} ->
-            file:read_line(FD), % skip  MemTotal
-            {ok, Data} = file:read_line(FD),
-            ["MemFree:",RawFree,_] = string:tokens(Data," \n"),
-            case file:read_line(FD) of
-                {ok, "MemAvailable:" ++Tail} ->
-                    [Avail,_] = string:tokens(Tail," \n"),
-                    file:close(FD),
-                    list_to_integer(Avail)/1024;
-                {ok, NextLine} ->
-                    ["Buffers:",Buf,_] = string:tokens(NextLine," \n"),
-                    {ok, LastLine} = file:read_line(FD),
-                    ["Cached:",Cached,_] = string:tokens(LastLine," \n"),
-                    file:close(FD),
-                    (list_to_integer(RawFree)+list_to_integer(Buf) + list_to_integer(Cached)) / 1024
-            end;
-        _ ->
-            0
+    case os:cmd("awk '/^MemFree|^Cached|^Buffers/ {freemem+=$2} END {print freemem}' /proc/meminfo") of
+        "" -> 0;
+        Data ->
+            RealFreeMem = string:strip(Data, right, $\n),
+            list_to_integer(RealFreeMem) / 1024
     end;
 get_os_data(freemem, {unix, sunos}) ->
     Result = os:cmd("vmstat 1 2 | tail -1"),