* Add 'pretty' target to reformat sources to be compliant with code style.
* Add 'pretty-check' target to check if sources are compliant with code style.
* Update travis-ci script to use 'pretty-check' and enforce code style.
* The 'pretty' and 'pretty-check' targets rely on the 'astyle' tool.
* posix: logging: Use `stderr` instead of `stdout` for debug logging.
* posix: serial: Restore socket `termios` settings for `stdin` and `stdout` at exit.