Restructure the way libpam-sys-impls works to fix cross-compilation. The previous structure of libpam-sys-impls meant that things got confusing (including for me) between what constants were build-time and what constants were run-time. This broke cross-compilation. This simplifies the way that works so that `libpam-sys-impls` has *no* build script itself and is intended mostly as a library to be included in other libraries' build scripts (while also exporting the PamImpl enum).

# 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