* Reopen log files after receiving a SIGHUP signal
Implements a system signals listener in log.FileLogWriter.
After receiving a SIGHUP signal from the system, the listener will close the current log file and then open it again.
The listener will finishes after receiving a SIGINT, SIGKILL or SIGTERM signal.
Closesgrafana/grafana#2497
* Move the SIGHUP handling to the main file