This fixes several issues with the current `uname` bindings:
- Do not ignore `uname` errors; at least on glibc `uname` can fail,
so now it returns a `Result` instead of assuming that the call
will always succeed.
- Do not assume `uname` will initialize every member of `utsname`;
not every implementation initializes every field, so internally
the struct is now zero-initialized.
- Do not blindly assume strings returned by `uname` will always be valid UTF-8;
`UtsName`'s accessors will now return `&OsStr`s instead of `&str`s.