Update bare pointers to incl. their mutability.

* `rust` master has replaced bare pointer (`*`) w/ `*mut` and `*const`
Integer literals no longer assume a width.

This code is equivalent to the original, specifying the `int` type.
This type is defined as being pointer-width.
Enums expect `int` but the `posix88` constants are `c_int` (aka `i32`s)
Allow `fn version()` to mutate version numbers in place.

The `rust` allocated memory for the version no. needs to be modified by FFI.

In light of recent changes: we mark the variables as mutable, and make sure
the FFI accepts mutable raw pointers.
setsockopt value does not have to be mutable
Merge pull request #38 from drbawb/feature/fix-bare-ptr

Update for rust-master: specify mutability of bare pointers, and width of integer literals.