kernel: event modification functions return previous value

Update the return value of functions that modify the internal event
state from `void` to `uint32_t`, so that calling code can determine
whether the event was already in a given state, or if the call modified
it.

This simplifies the usage of `struct k_event` as an alternative to
`atomic_t` that users can block on.

Implements #57216

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>