dns plugin: Implement the `SelectNumericQueryTypes' option.

dns plugin: Implement the `SelectNumericQueryTypes' option.

I've added this patch to my collectd because I have my data dir overpopulated
by unknown query types (those not found in nameserv.h which are converted to
their #xxxx numeric form).

When enabled in configuration file, this option just prevent registering these
qtypes to the list.

I don't see any issues, so I decided to contribute this patch to the list.

This has being tested against 4.6.2, but shouldn't have problems with 4.7.0

Your comments are welcome.

Mirko

Signed-off-by: Florian Forster <octo@huhu.verplant.org>