add synchronization to stdin/stdout/stderr

[japaric]

The current implementation is not synchronized. This is OK for now because we don't have threads. Once we get threads the implementation will have to be updated. The code that uses synchronization is already there but we haven't switched to it yet.

This is blocked by TLS support. cc #30

[tbu-]

Not thread-safe is the wrong word. It is thread-safe (as in memory-safe), but lines from different threads can be interleaved.

[japaric]

You are right. Changed thread-safe to synchronization.

[tbu-]

Let's make this the tracking issue for io::stdout.