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 - Replace outdated PAM doc links with man7.org pages.

auth sufficient pam_http.so url=http://localhost:3000
account sufficient pam_http.so