include/mutex,wmutex: Add weak mutexes that don't enforce ownership

include/mutex,wmutex: Add weak mutexes that don't enforce ownership

When using mutexes as a means for signalling between two processes, the
lock and unlock operation will almost never be executed by the same
process. However, the mutex module does not allow such behavior.
This commit adds the wmutex module (the "w" is for "weak") which does
not enforce ownership on mutexes.

As a side note, the same could have been achieved by adding something
like mutex_weak_lock() and mutex_weak_unlock() to the mutex module, but
that would lead to people mixing the two. Implementing them in separate
modules hopefully makes it more clear that using wmutex functions on a
mutex (and vice-versa) is not acceptable. If necessary, I might even
implement the two in different ways so a mutex can't be released with
wmutex_unlock().