[SWP] coap plugtests

This contains the coap-plugtests application. It passes the sections "Base CoAP", "Block", and "Link format" as specified here: https://github.com/cabo/td-coap4/

It depends on a patched libcoap that is referenced in the pkg/libcoap/Makefile and can be found here: https://github.com/sgso/libcoap/tree/riot-pkg. The patches are based on the current HEAD of libcoap. I worked with upstream to include most of the patches that are currently included in the libcoap pkg directory (most of them were due to RIOTs pedantic compiler flags).

There are no handlers programmed for the "Observe" part of the plugtest but my libcoap branch proposes a variant with timer enabled socket functions to compensate for the missing select/*poll.
The idea is that the programmer has to think about why the parameter is unused. Ideally the `(void) x;` is accompanied by a comment. Suppressing `-Wunused-parameter` with a global pragma is a no-go.