logind: rework session creation logic, to be more reusable for varlink codepaths

logind: rework session creation logic, to be more reusable for varlink codepaths

This separates the preparatory checks that generate D-Bus errors from
the code that actually allocates the session. This make the logic easier
to follow and prepares ground so that we can reuse the 2nd part later
when exposing session creation via Varlink.