Debian :: Commands To Change Authentication Information

I wanted to change authentication options in
1) /etc/pam.d directory files common-password, common-auth, common-account
2) /etc/login.defs - PASS* fields

I know the fields that need to be changed but what I could not find was
a) commands that change these options similar to useraddusermod and so on but those which can change system wide settings
b) API that can change these options (C API)

I have almost accepted that there in fact aren't such things. You have to manually update the files. I would prefer not though.So I am asking here as last attempt to find such commandsAPI. Please let me know if they are available.