[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.
I could create a new patch set on top of the current version (like we do now with the older version) which seems to be in a good state and hasn't changed since August/September. Should libcoap development pick up some momentum in the future I'd recommend to ask its maintainer to include the patches together with a RIOT flag rather than fork it. But currently it's not worth the effort, in my opinion.