Move sockent_client_disconnect above sockent_client_disconnect so we can use it there
Add ReconnectInterval option to Network plugin

The Network plugin only performs DNS resolution at initialization. This
can be problematic when trying to performs migrations of collectd
infrastructure or when trying to create HA solutions which are dependant
on DNS.

The ReconnectInterval options forces a reconnect of all the sockets at
the defined number of seconds. By default no re-connections are attempted
if this option is not set.
openvpn plugin: Don't signal an error when no clients are connected.

In the multi1_read() function, an error (zero) was returned when no
clients were currently connected to the OpenVPN server, because the
"read" variable was initialized to zero and the while loop exited before
it was set to one. This is not the intended behavior.

Thanks to @srix for reporting this issue!

Fixes: #731