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 - Replace outdated PAM doc links with man7.org pages.

all:
    cargo build

install:
    @cargo build --release
    sudo cp conf/sober-auth /etc/pam.d/
    sudo cp ../target/release/libpam_sober.so /lib/security/pam_sober.so

test:
    @just install
    gcc -o ../target/pam_test test.c -lpam -lpam_misc