run: Process::new() should take &[Str] instead of &[~str] for args

Due to an ICE (filed as #8835), run::process_status() still takes a
&[~str].

Fixes #7928.
auto merge of #8203 : kballard/rust/process-new-args, r=cmr

Fixes #7928.

As a side-effect, this no longer clones all the arg strings on unix.