Give MVMOSHandle a mutex slot.

We need to protect the various IO handles from multiple threads doing
input/output operations.
First pass at locking for syncstream.

Deals with the immediate issues when multiple threads 'say' at once,
but we really need to take the time to do a systematic pass through IO
and the things it calls (like decode stream) and make sure we never
get into a situation where we lose a mutex.
Avoid a hang in semtryacquire.

This in turn caused Rakudo to hang once the thread pool scheduler had
spawned its maximum number of threads.