tcpconns/linux: Use netlink instead of parsing /proc/net/tcp{,6}
little style changes (code review)
tcpconns plugin: Only chose between netlink / proc on the first iteration.

Rather than going for netlink each iteration and fall back to /proc, try
this on the first iteration only and stick with the result afterwards.
Also contains some improvements to the error handling / reporting.
Plugin notify_snmp. Send notifications as snmp traps.