Add support for pam_get_authtok and minor cleanups. This change adds the pam_get_authtok function for PAM modules, as well as performing a few cleanups: - Pattern match in a few more places. - Pull out string-copying into a function. - Format and run clippy. - Replace outdated PAM doc links with man7.org pages.

# Compiled files
*.o
*.so
*.rlib
*.dll

# Executables
*.exe

# Generated by Cargo
target/
Cargo.lock

# Override top-level .gitignore in Misc repo
!src/tozny