Where does MUTEX_INITIALIZER come from on Hurd?

[Florian Weimer]

We currently have malloc-machine.h and libc-lock.h.  For NPTL targets,
malloc-machine.h provides MUTEX_INITIALIZER.  But Hurd malloc-machine.h
does not provide it, and libc-lock.h references it.  Where does it come
from?  The generic definition (0) does not look like it would work on
Hurd because the lock is a struct.

The libc-lock.h wrappers currently do not seem to provide *any*
replacement for MUTEX_INITIALIZER.  Should we assume that
zero-initialization works for mutexes?

Florian