Handle kernel_pid_t consistently

This was missing after #1374 got merged - otherwise, e.g., 6LoWPAN is not working.

KERNEL_PID_NULL is a negative number. If kernel_pid_t variables are
initialized to this value, one have to check for lt/gte 0.
I think it's cleaner if every global variable of this type gets initialized to the correct value - if it might be not necessary in some cases.