Fix race condition in ts_utils:make_dir/1

[tisba]

There is a race condition, when multiple Erlang VMs are started almost simultaneously which is a typical case if you are not using tsung's SMP option.

I observed it a couple of times now, that ts_local_mon failed to write the protocol_local dumpfile. The reason is that the check and set is not one atomic operation in ts_utils:make_dir_rec/3. There is a window after the check, when another process could have created the directory already, resulting in a {error, eexist}.

In #88 the check for ok in ts_local_mon:init/1 was wrongfully removed (I'm sorry for that). Back then I noticed the {error, eexist} and assumed that this is a result from the fact that the entire log directory already existed, which is not the case. ts_utils:make_dir/1 and ts_utils:make_dir_raw/1 should now be better protected against the mentioned race condition.