Fix sync and whoami on windows.

Fixed compile errors on MinGW, mainly mutable reference errors.

In sync.rs, for some reason "use std::ptr::null" doesn't work unless it's inside the windows "mod platform {" block - is that a compiler bug or defined behavior?

There are still some deprecated code warnings in sync.rs concerning the use of str::to_c_str, but I'm not sure how to fix them without resorting to transmute