logind: Add a two new variables to the user session tracking file.

This counts 'online sessions' in addition to 'active sessions' and 'sessions'.

In this context, an 'online session' covers all session in the 'active' state
in addition to the explicit 'online' state.

This provides an easy machanism to determin all relevant sessions easily
(i.e. those that are not 'closing') and adds new semantics to the sd-login.c
APIs sd_uid_get_sessions() and sd_uid_get_seats() where the require_active
argument can be supplied as a value 2 which only lists sessions which are
'online'.

This functionality should allow client applications to avoid deadlocks where
they only exit when all sessions are complete, such as a the problem where
PulseAudio will not exit until all sessions are gone, but in itself prevents
the session from exiting.