Improve mutex debugging prints

Improve mutex debugging prints

When lock contention is detected, print also the name of the thread
holding the lock and also where the lock was tried to acquire.

After the lock is finally acquired after the contention, add a new
print there as well to indicate that the lock is now acquired and the
thread will continue.

This adds overhead to the mutex operations, but that shouldn't be an
issue yet. If it becomes too slow, an option can be added later to
remove the prints and the overhead altogether. Now we are more happy
about the improved verbosity of the locking operations.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>