Add initialization options to RustBox.

In particular, the BufferStderr option causes RustBox to automatically
redirect stderr to an internal pipe, flushing it when RustBox exits.
This means that it is very easy for external programs to "do the right
thing" on panics, without having to hook into rustrt at all.  Almost all
resource management has also been delegated to internal RAII guards, so
it should be relatively easy to modify going forward without worrying
about leaking.