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.

[package]
name = "libpam-sys-helpers"
version.workspace = true
authors.workspace = true
repository.workspace = true
edition.workspace = true
rust-version.workspace = true
license.workspace = true
readme = "README.md"

[dependencies]
libc = "0.2"

[build-dependencies]
libc = "0.2"