util: enable getpid to be replaced This will enable tests to write stable process ids.

blackbox: refactor use of vfs as _bbvfs

blackbox: flush output file descriptor Without this, when there are multiple ui views, each blackbox will have its own file handle, and the logging will be in a really bad order. Also, because of the way blackbox works, it never closes its file handles, which means the last output before exit is often lost.