Add module-specific data support. This adds support for a safe form of `pam_get_data` and `pam_set_data`, where data is (as best as humanly possible) type-safe and restricted to only the module where it was created.

use libpam_sys_consts::pam_impl;

fn main() {
    println!("cargo:rustc-link-lib=pam");
    pam_impl::enable_pam_impl_cfg();
}