pam: add session class "none" to disable logind sessions (#35171)

pam_systemd is used to create logind sessions and to apply extended
attributes from json user records. Not every application that creates a
pam session expects a login scope, but may be interested in the extended
attributes of json user records. Session class "none" implements this
service by disabling logind for this session altogether.

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Closes: #34988

diff --cc man/pam_systemd.xml
Simple merge

diff --cc src/login/logind-dbus.c
Simple merge