Stop statting paths before opening them

[pcwalton]

https://github.com/rust-lang/rust/blob/master/src/libstd/fs/mod.rs#L275-L280

It's ruining my ability to write a sandbox. The stat syscall might be blacklisted while open is not. That should not prevent you from opening files!

Nominating because I believe this must be fixed before 1.0.

[pnkfelix]

1.0, P-high, needs-decision.