We need a getenv() that returns ~[u8]

[lilyball]

We need a variant of os::getenv() that returns a ~[u8]instead of astr. Or more usefully,getenv()itself should return[u8]and perhaps we should have agetenv_str()variant that returns~str`.

I think arguments needs the same treatment too.

[huonw]

Seems to be very similar to #7188, even down to the bug filer. ;P

[lilyball]

D'oh! I searched for getenv, which unfortunately doesn't show up in that older one.