Handle kernel_pid_t consistently

This is a cleanup for #1374 got - without this PR, e.g., 6LoWPAN is not working.

KERNEL_PID_NULL gets renamed to KERNEL_PID_UNDEF and is a negative number. `kernel_pid_t`  variables should be initialized to this value and checked accordingly.
Ah, wrong line, the one below is the one I meant.