Add std::os::set_errno as a counterpart to std::os::errno

This pull request adds a `std::os::set_errno` function to allow rust to manipulate the thread's `errno` value.  In my particular case, I am needing to do this when interacting with a c-library I am wrapping.

Note I don't have rust building on a Windows platform currently so have been unable to test my implementation for `[cfg(windows)]`
Note that my above comment is predicated on `std` only exposing `errno` as a read-only value, and `libc` exposing `errno` as a read/write value using `c_int`, as stated in my previous comment.