Add support for /etc/smcroute.d/*.conf

[troglobit]

Often a user wants to add routes from multiple sources, using perhaps different tools. Editing /etc/smcroute.conf is not sustainable.

Also, on SIGHUP today (to reload smcroute.conf) the daemon closes the routing socket to flush all routes before adding the ones from the config file. This means unchanged routes that exist before and after SIGHUP will experience a drop in their given flows, and packet counters are reset.

Hence, when adding support it is required to also add a diff engine to track what changes actually need to be done, to not disturb established flows.