* 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.

Closes grafana/grafana#2497

* Move the SIGHUP handling to the main file