FreeBSD port

[wezm]

Building on FreeBSD results in this error as there is no os module implementation for it.

error[E0432]: unresolved import `os::get_input`
  --> src/main.rs:55:9
   |
55 |     use os::get_input;
   |         ^^^^^^^^^^^^^ no `get_input` in `os`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
error: could not compile `what`.

[imsnif]

If you'd be willing to do some testing (I don't have access to a FreeBSD box unfortunately), this might be a quick fix (I think trying either the linux or the mac version with it might do the trick).