Currently it is not possible to check if a message was sent over a bus
or if it was send the usual way using `msg_send()`.
This adds a flag to the `sender_pid` if the message was sent over a bus.
`MAXTHREADS` is currently set to 32, so there is still plenty of room in
the PID space. (`kernel_pid_t` is `int16_t`)
The message type for bus message type is already accessed through a getter
function, so it's just consistent to do the same for sender_pid.
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