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)]`
@thestinger C defines `errno` as something "which expands to a modifiable lvalue that has type **int**". `int` is not a fixed-width size, it is an architecture- and compiler-defined size.