core: introduce `msg_sent_by_int()`

msg_send_int() sets `m->sender_pid = target_pid`. This was used to flag a
message as having been sent by an ISR.

This PR introduces a static inline function `msg_sent_by_int()` and a
specific define for this purpose.
Merge pull request #1791 from Kijewski/msg_send_int-typo

core: introduce `msg_sent_by_int()`