powerdns plugin: Replace the `Command' option with the `Collect' option.

The original code let the user define the command to use when querying the
server. This commit changes that and lets the user select the _values_ he's
interested in. Depending on whether an authorative server or a recursor is
collected this
- issues a `SHOW *' and manually greps the interesting pieces, or
- builds a command that requests only the interesting values.

Code compiles but it untested. Manpage not yet updated.