Patch MiniSandbox to support i686 platform and improve integrity

This patch proposes to fix the following issues with rayage.MiniSandbox,

1. system call number of `open()` was hard-coded as `2`, which is correct as per the 64bit ABI of `x86_64`, but incorrect for `i686`;
2. use of `ptrace.debugger.PtraceProcess` was unnecessary if the sole purpose was to extract the path argument of `open()`; use `Sandbox.dump(T_STRING, address)` instead; moreover, attaching to the sandboxed process may change process hierarchy and potentially invalidate some internal states of the sandbox;

DISCLAIMER: I am the author of `libsandbox`.