netapp plugin: Changed some type names.

netapp plugin: Changed some type names.

The structure is roughly like this: Structs that only hold flags to tell
the functions what data to submit are prefixed with "cfg_". Structs that
hold old values for counters are prefixed with "data_".

The "disk_t" type now included flags, too, to indicate valid / invalid
values. The "query_submit_disk_data" function has been changed to honor
those flags.

Various "volume_data" stuff has been renamed to "volume_usage" to make
it more distinguishable from "volume_performance".

Various defines are now also prefixed with "CFG_" to show which flags
are used for configuration and which are used do mark counters valid.
The latter use the "HAVE_" prefix.