netmgr: refactor isc__nm_incstats() and isc__nm_decstats()

After support for route/netlink sockets is merged, not all sockets will have stats counters associated with them, so it's now necessary to check whether socket stats exist before incrementing or decrementing them. rather than relying on the caller for this, we now just pass the socket and an index, and the correct stats counter will be updated if it exists.