gnrc_priority_pktqueue: simplify API

[miri64]

There is a lot simplification possible, if you do not add the pool of nodes to the queue and let the "user" handle it externally. One would now go to allocate a node like this (sometimes a node allocated somewhere in a function allocated might already be enough for an application so a central array would be overkill). Also there are a lot of style issues that I did not fix with this PR that need fixing before we can merge RIOT-OS#5950!

With this change one would now push a packet pkt to the queue q as follows:

gnrc_priority_pktqueue_node_t *some_node = get_somewhere();
priority_pktqueue_node_init(some_node, priority, pkt);
priority_pktqueue_push(&q, some_node);

Style-note: the functions need gnrc_ prefixing of course ;-)

[zhuoshuguo]

Thanks a lot! Martine @miri64 , so the idea is to allocate the node externally by the API user itself.

[miri64]

Yapp, like in the rest of RIOT ;-)