core: rename tcb_t -> thread_t, move into thread.h

core/include/debug.h

core/include/sched.h

core/include/tcb.h

core/include/thread.h

core/msg.c

core/mutex.c

core/sched.c

core/thread.c

core/thread_print_msg_queue.c

cpu/atmega2560/periph/timer.c

cpu/atmega_common/thread_arch.c

cpu/lpc11u34/periph/gpio.c

sys/cpp11-compat/condition_variable.cpp

sys/include/pipe.h

sys/pipe/pipe.c

sys/posix/pthread/include/pthread_rwlock.h

sys/posix/pthread/pthread_barrier.c

sys/posix/pthread/pthread_cond.c

sys/posix/pthread/pthread_rwlock.c

sys/ps/ps.c