network plugin: Fix an invalid size of buffer being used.

network plugin: Fix an invalid size of buffer being used.

When specifying the amount of data to copy, we used `sizeof (buffer)' where
`buffer' is a pointer, giving 4 or 8 bytes, depending on the architecture (and
not depending on the actual buffer size). This results in the `type' being sent
much more often than necessary and sometimes not sending a new type when is was
actually necessary. The only prominent case in the default configuration(s) was
`cpufreq' being used instead of `cpu', though.

While in the process, the global `type' buffer was replaced, because an
appropriate buffer is in `value_list_t' now.

Much thanks to Bruno Prémont for reporting and debugging this issue :)

References: #37