pam_systemd: split pam_sm_open_session() into more digestable blocks

pam_systemd: split pam_sm_open_session() into more digestable blocks

Let's separate four different parts of pam_sm_open_session():

1. Acquiring of our various parameters from pam env, pam data, pam items
2. Mangling of that data to clean it up
3. Registering of the service with logind
4. Importing shell credentials into environment variables
5. Enforcement of user record data

This makes the code a lot more readable, and gets rid of an ugly goto
label.

It also corrects things: if step 3 doesnt work because logind is not
around, we'll now still do step 4, which we previously erroneously
skipped.

Besides that no real code changes.