Switch to OpenBSD fopen/fclose.

This means all our stdio implementation is now the OpenBSD implementation.
The only thing we lose is the STDIO_THREAD_LOCK calls but they were no-ops
anyway.

We should probably talk to upstream about this. Either fix the locking or,
preferably, encourage them to move away from this pooling (especially since
there's no eviction policy).

Bug: 17154680
Change-Id: Ie2523e444a7d0965b8d141d57e3e11f6432d5b9a