netmgr: refactor isc__nm_incstats() and isc__nm_decstats()

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.