[riot-notifications] [RIOT-OS/RIOT] core/mutex: Add mutex_cancel (#15442)

[Marian Buschsieweke]

> but, what happens when the lock gets canceled anyways? (locker uses "mutex_lock()", but someone calls "mutex_cancel()" on it?

`mutex_lock()` must never fail :-)

Btw: I think I will place a preconditions that `mutex_lock_may_fail()` is called with IRQs disabled. This would get rid of all the complexity, as one could simply do:

```C
   setup_isr_that_unblocks_mutex_but_mask_it();
   unsigned state = irq_disable();
   unmask_isr_that_unblocks_mutex();
   if (mutex_lock_may_fail(&mut, state) == 0) {
       work_on_shared_data();
       mutex_unlock();
   }
```

Here there is no chance that the ISR hits right before the call to `mutex_lock_may_fail()`, and all those abuse `wait_data` to signal cancellation is no longer needed. So not even a pseudo-module would be required to enable it, as the linker would be totally capable to garbage collect every single byte of RAM and ROM needed by this.

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