hsflowd

Add new command-line option "-D logFile" for debug logging to append to logFile instead of stdout. This logFile is reopened on SIGHUP so it can be used with standard logrotate(1) options if needed.
Receiving SIGUSR2 now increments the debug log level.

Example:
If hsflowd is started with "hsflowd -d -D /var/log/hsflowd.log"
then the logs can be followed with "tail -f /var/log/hsflowd.log"
and the debug level can be boosted with "killall -USR2 hsflowd"