Add c_str::CString.as_bytes_no_nul()
Use str::from_utf8_lossy() in os::args(), add os::args_as_bytes()

os::args() was using str::raw::from_c_str(), which would assert if the
C-string wasn't valid UTF-8. Switch to using from_utf8_lossy() instead,
and add a separate function os::args_as_bytes() that returns the ~[u8]
byte-vectors instead.
Use str::from_utf8_lossy() for os::env() and friends

Parse the environment by default with from_utf8_lossy. Also provide
byte-vector equivalents (e.g. os::env_as_bytes()).

Unfortunately, setenv() can't have a byte-vector equivalent because of
Windows support, unless we want to define a setenv_bytes() that fails
under Windows for non-UTF8 (or non-UTF16).
auto merge of #12283 : kballard/rust/env-args-bytes, r=erickt

Change `os::args()` and `os::env()` to use `str::from_utf8_lossy()`.
Add new functions `os::args_as_bytes()` and `os::env_as_bytes()` to retrieve the args/env as byte vectors instead.

The existing methods were left returning strings because I expect that the common use-case is to want string handling.

Fixes #7188.