feature/single_user Allow only one user to access serial driver

feature/single_user Allow only one user to access serial driver

- use fcntl() to switch file access between non-blocking and blocking
  while flushing ringbuffer, remove separate open()/close() call
- count open references in serial driver and deny access if already opened
- fix memory leak when app is unable to access the serial driver
- print informative messages when app is unable to access the serial driver
- added fclose() to fix memory leak
- added async version of disable heartbeat