Some improvements to avoid the deadlock in lsl

* Increate stack size to 1024*1024 bytes (as shown in the example on the
  clone man page)
* Don't try to free the stack manually
* Switch to std::mutex and use lock_guards
* Create the mutex in the child process that creates the thread later on
  (instead of a global mutex which is also visible in the initial
  process)

Make latest changes for release:

- rename to lsl
- add some security options (dropping of capabilities and seccomp)