Turn everything into OsString and Just Walk Out! for strings with nul. To reduce the hazard surface of the API, this replaces most uses of &str with &OsStr (and likewise with String/OsString). Also, I've decided that instead of dealing with callers putting `\0` in their parameters, I'm going to follow the example of std::env and Just Walk Out! (i.e., panic!()). This makes things a lot less annoying for both me and (hopefully) users.

# The `libpam-sys` tests

This crate exists to provide tests for `libpam-sys`, without being part of it and having all its dependencies come along.
It works like [the `libc-test` crate][libc-test] for `libc` (and also uses `ctest`).

The `build.rs` script generates test code which is included by files in `tests/`.

[libc-test]: https://github.com/rust-lang/libc/tree/ctest-v0.4.11/libc-test