configure_pam_on_node: Drop user_list loop

Drop the loop that constructs $user_list, and just use root,adm for the
polyinstantiation blacklist for /tmp and /dev/shm.

Add coments to the relevant code.
Merge pull request #36 from Miciah/configure_pam_on_node-drop-user_list-loop

configure_pam_on_node: Drop user_list loop