Investigate and document thread safety requirements

[tanriol]

According to pam(3)

The libpam interfaces are only thread-safe if each thread within
the multithreaded application uses its own PAM handle.

Looks to me like the current implementation either is unsound due to violating libpam thread-safety requirements or relies on some internal libpam implementation details for soundness.

[ChocolateLoverRaj]

Is it even possible to have each thread use its own PAM handle?

[ChocolateLoverRaj]

If unsafe_send.rs was deleted and the solution in #6 (comment) was implemented would it meet PAM's thread safety requirements?

Also PAM should've been written in Rust so we wouldn't have to read documentation and could just confidently write safe code as long as we didn't use the word unsafe in this code.