Recursive mutexes?

[Marius Vollmer]

Our current coop mutexes are not 'recursive'.  This means that a
thread blocks when it tries to lock a mutex that it already has
locked.

I think we should make our mutexes be recursive by default.  Expecting
to block when locking a mutex that is already lcoked by one self is
not very useful, since no one can unlock that mutex (excepts asyncs).

The only good argument against recursive mutexes that I can think of
is a tiny performance gain since you don't need to do some checking.

SRFI-18 specifies non-recursive mutexes and allows non-owning threads
to unlock a mutex.  Such uses of a mutex are, in my view, a mockery of
condition variables should be avoided.

If non-recursive mutexes turn out to be important, we can provide them
as well, as an option.

Ok?

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