Add argument to thread_create
core: remove extra thread_create_arg() function
Merge pull request #856 from Kijewski/issue-847

 core: add an argument to `thread_create()` (implementation)
sys/transceiver: fix t9r_register API compliance
sys/transceiver: implement transceiver_unregister
sys/transceiver: refactor un/register

remove race conditions
make less restrictive (allow double-un/registers)
make loops more readable