pthread fast mutexes

[Kevin Ryde]

In the current cvs on my i386 debian,

        (unlock-mutex (make-mutex))

gives a seg fault.

I'm guessing it's because make-mutex uses a pthread fast mutex, and
that style means no error checking by the unlock.

Maybe the scheme level functions will need to use the "normal" mutexes
so the above can throw an exception.  Or is there some other theory on
this?