PR 5432: Thread safety patches

[Benjamin Kosnik]

> If recursive action is required (and it appears so at first glance),
> then I can build us a portable recursive mutex from the abstraction
> provided by gthr.

Cool.

> > Sentry work is concerned with sharing particular iostream objects 
> > among threads, where Craig's work is necessary to allow sharing the 
> > iostream library itself among threads.  IOW, I think Craig's changes 
> > are completely orthogonal to this work, fix a serious bug, and ought 
> > to go in *immediately*.  Arguably it should go into 3.0.x too, if we've 
> > advertised 3.0 as thread-safe-as-long-as-you-lock-shared-objects.
> 
> Fully agreed.

Ok. What do you think about

testsuite/optional/threads

?

It doesn't have to be used right away, but I think it's best to start 
saving good test cases now.

-benjamin