Use mktemp for temporary download directory

Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.

The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
Fall back to hard coded download directory

If mktemp fails, fall back to a hard coded directory, per @nodakai's feedback.
Merge branch 'master' into cfg_tmp_dir

Conflicts:
	src/etc/rustup.sh
Better temporary directory name
Split overly long line
librustc: Traverse arbitrarily deep for nullable enum opt.
librustc: Try looking in tuple fields for nullable enum opt.
librustc: Try looking in fixed sized arrays for nullable enum opt.
Add tests.
librustc: Add NonZero lang item and use it if possible for nullable pointer enum opt.
libcore: Add NonZero lang item and implement some methods.
libcollections: Use NonZero in Vec.
liballoc: Use NonZero in Rc.
liballoc: Use NonZero in Arc.
libcore: Make it unsafe to create NonZero and impl Deref.
Add tests for NonZero.
Don't expose NonZero through libstd.
libcore: Don't impl RawPtr* traits for NonZero.
librustc_trans: Get rid of unnecessary allocation in finding discriminant field.
libcore: Use Zeroable trait to try to limit what types may be used with NonZero.