Make env: return ENOENT on non-existent; support unlink()

This also requires a small modification to libstd; I guess I'll submit a PR to redox-os/rust, and if it looks okay a PR can be sent upstream as well. Everything has to be rebuilt for this change, if it sets environmental variables. That seems unavoidable though.

Hopefully this is okay, but I don't know why that if condition was commented out, so perhaps it causes issues I'm not aware of.