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

This fixes two breaking changes from the `rust` nightlies.

* [Integer literals no longer have a default width.](https://github.com/rust-lang/rust/commit/a5bb0a3a4574af88add700ace7aefc37172fa7a5)

* [`*T` has been removed from the language in favor of `*mut T` and `*const T`](https://github.com/rust-lang/rust/pull/15208)

  * I've elected to use `*const T` unless the pointer is passed to a callee which expects an `&mut` reference

These changes allow me to continue building `rust-zmq` on the following version of `rustc`:
~~`rustc 0.11.0-pre (b569c77148e8e839b38b678c7c167efc643d2721 2014-06-29 11:21:39 +0000)`~~
`c5e2fe4` tested on `rustc 0.11.0-pre (566b57f3de7de80599b052b1a042eef8c9091e7f 2014-06-30 12:16:30 +0000)`
`optval` should have a type `*const c_void`.